Blockchain is a kind of distributed ledger. A blockchain is an independent ledger, and two different chains are not related to each other. Therefore, in essence, there is no way to transfer the value between the ledger. However, for a specific user, the value stored by the user on one blockchain needs to be transferred to the value on another chain, which is cross-chain asset circulation. Cross-chain asset circulation allows value to flow directly across the barriers between chains. Cross-chain is essentially the same as currency exchange. Cross-chain does not change the total value of each chain, but only an exchange between different holders. Further, a cross-chain distributed transaction is one in which multiple steps of a transaction are distributed across different blockchains and require consistency of the transaction. This is an extension of cross-chain asset flow, extending the behavior of asset exchange into arbitrary behavior. Cross-chain distributed transactions make cross-chain smart contracts possible. A smart contract can be executed on multiple different blockchains, either all executed or all returned to the state before execution. Cross-chain smart contracts will break the information island effect between chains and greatly expand the application scenarios of blockchain.