Implementations of this specification provide a method and an apparatus for transaction processing in a service blockchain. An example method performed by a node device in the service blockchain includes receiving a target transaction, from a service system interlinked with the service blockchain; in response to receiving the target transaction, (i) triggering execution of a target service and (ii) determining a target asset blockchain corresponding to the target service from a plurality of asset blockchains interlinked with the service blockchain through a cross-chain relay; invoking, through the cross-chain relay, an asset exchange service deployed on the target asset blockchain, to complete asset exchange processing between transaction accounts related to the target transaction; obtaining, through the cross-chain relay, an asset exchange credential between the transaction accounts related to the target transaction; and after obtaining the asset exchange credential, performing further transaction processing on the target service.