diff --git a/core/src/lib.rs b/core/src/lib.rs
index 0743cd3ab81..82315f56039 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -18,7 +18,7 @@ pub mod uncle;
 pub use crate::error::Error;
 
 pub use bytes::Bytes;
-pub use occupied_capacity::{capacity_bytes, Capacity};
+pub use occupied_capacity::{capacity_bytes, Capacity, OccupiedCapacity};
 pub type PublicKey = numext_fixed_hash::H512;
 pub type BlockNumber = u64;
 pub type EpochNumber = u64;
diff --git a/miner/src/block_assembler.rs b/miner/src/block_assembler.rs
index d3d3934b9fa..f27618a323e 100644
--- a/miner/src/block_assembler.rs
+++ b/miner/src/block_assembler.rs
@@ -9,14 +9,14 @@ use ckb_core::transaction::{
     CellInput, CellOutput, ProposalShortId, Transaction, TransactionBuilder,
 };
 use ckb_core::uncle::UncleBlock;
-use ckb_core::BlockNumber;
+use ckb_core::OccupiedCapacity;
+use ckb_core::{BlockNumber, Bytes, Capacity};
 use ckb_core::{Cycle, Version};
 use ckb_logger::{error, info};
 use ckb_notify::NotifyController;
 use ckb_shared::{shared::Shared, tx_pool::ProposedEntry};
 use ckb_store::ChainStore;
 use ckb_traits::ChainProvider;
-use ckb_verification::TransactionError;
 use crossbeam_channel::{self, select, Receiver, Sender};
 use failure::Error as FailureError;
 use faketime::unix_time_as_millis;
@@ -403,19 +403,21 @@ impl<CS: ChainStore + 'static> BlockAssembler<CS> {
     ) -> Result<Transaction, FailureError> {
         let witness = lock.into_witness();
         let input = CellInput::new_cellbase_input(candidate_number);
-        let output = CellOutput::new(
-            self.shared
-                .consensus()
-                .genesis_epoch_ext()
-                .block_reward(candidate_number)?,
-            self.config.data.clone().into_bytes(),
-            self.shared.consensus().bootstrap_lock().clone(),
-            None,
-        );
-        // User-defined data has a risk of exceeding capacity
-        if output.is_lack_of_capacity()? {
-            return Err(TransactionError::InsufficientCellCapacity.into());
-        }
+
+        let block_reward = self
+            .shared
+            .consensus()
+            .genesis_epoch_ext()
+            .block_reward(candidate_number)?;
+
+        let lock = self.shared.consensus().bootstrap_lock().clone();
+
+        let r#type = None;
+
+        let data = self.custom_data(&block_reward, &lock, &r#type)?;
+
+        let output = CellOutput::new(block_reward, data, lock, r#type);
+
         Ok(TransactionBuilder::default()
             .input(input)
             .output(output)
@@ -429,16 +431,13 @@ impl<CS: ChainStore + 'static> BlockAssembler<CS> {
 
         let witness = lock.into_witness();
         let input = CellInput::new_cellbase_input(candidate_number);
-        let output = CellOutput::new(
-            block_reward,
-            self.config.data.clone().into_bytes(),
-            target_lock,
-            None,
-        );
-        // User-defined data has a risk of exceeding capacity
-        if output.is_lack_of_capacity()? {
-            return Err(TransactionError::InsufficientCellCapacity.into());
-        }
+
+        let r#type = None;
+
+        let data = self.custom_data(&block_reward, &target_lock, &r#type)?;
+
+        let output = CellOutput::new(block_reward, data, target_lock, r#type);
+
         Ok(TransactionBuilder::default()
             .input(input)
             .output(output)
@@ -446,6 +445,27 @@ impl<CS: ChainStore + 'static> BlockAssembler<CS> {
             .build())
     }
 
+    fn custom_data(
+        &self,
+        reward: &Capacity,
+        lock: &Script,
+        r#type: &Option<Script>,
+    ) -> Result<Bytes, FailureError> {
+        let mut data = self.config.data.clone().into_bytes();
+
+        let data_max_len = (reward.as_u64()
+            - lock.occupied_capacity()?.as_u64()
+            - reward.occupied_capacity()?.as_u64()
+            - r#type.occupied_capacity()?.as_u64()) as usize;
+
+        // User-defined data has a risk of exceeding capacity
+        // just truncate it
+        if data.len() > data_max_len {
+            data.truncate(data_max_len);
+        }
+        Ok(data)
+    }
+
     fn prepare_uncles(
         &self,
         current_epoch_ext: &EpochExt,
diff --git a/resource/ckb.toml b/resource/ckb.toml
index 513be554f6c..793133559f3 100644
--- a/resource/ckb.toml
+++ b/resource/ckb.toml
@@ -117,13 +117,14 @@ cell_output_cache_size  = 128
 #
 #     ckb cli secp256k1-lock <pubkey>
 #
-# Also, ckb allows the miner to add any data to the cellbase that he has dug out.
-# The data must be A 0x-prefixed hex string. **Please note** that the data cannot be
-# larger than the capacity value of the current cellbase. If it is larger,
-# the block template cannot be obtained normally. That is, it is impossible to mine normally.
+# Also, ckb allows the miners to add any data to the cellbase that they have dug out.
+# The data must be A 0x-prefixed hex string.
 #
 # note: The data field is optional.
 #
+# **WARNING**: if data is larger than the capacity value of the current cellbase,
+# it will be truncated
+#
 # Below is an example block assembler configuration section:
 #
 # [block_assembler]
diff --git a/test/src/specs/mining/bootstrap.rs b/test/src/specs/mining/bootstrap.rs
index 88419818022..acdbb5667eb 100644
--- a/test/src/specs/mining/bootstrap.rs
+++ b/test/src/specs/mining/bootstrap.rs
@@ -3,6 +3,7 @@ use ckb_app_config::{BlockAssemblerConfig, CKBAppConfig};
 use ckb_chain_spec::ChainSpec;
 use ckb_core::block::Block;
 use ckb_core::script::Script;
+use ckb_core::Bytes;
 use jsonrpc_types::JsonBytes;
 use log::info;
 use numext_fixed_hash::{h256, H256};
@@ -44,7 +45,9 @@ impl Spec for BootstrapCellbase {
 
         let blk: Block = node.rpc_client().get_block(hash).unwrap().into();
         assert!(
-            blk.transactions()[0].is_cellbase() && blk.transactions()[0].outputs()[0].lock == miner
+            blk.transactions()[0].is_cellbase()
+                && blk.transactions()[0].outputs()[0].lock == miner
+                && blk.transactions()[0].outputs()[0].data == Bytes::from(vec![1; 30])
         )
     }
 
@@ -66,7 +69,7 @@ impl Spec for BootstrapCellbase {
             config.block_assembler = Some(BlockAssemblerConfig {
                 code_hash: h256!("0xa2"),
                 args: vec![],
-                data: JsonBytes::default(),
+                data: JsonBytes::from_bytes(Bytes::from(vec![1; 30])),
             });
         })
     }