To deal with the phenomenon of "information and value islands" caused by the lack of interoperation among the increasingly emerging blockchain systems， a federated?autonomy?based cross?chain scheme was proposed. The elemental idea of this scheme is to form a relay alliance chain maintained by participated blockchain systems using blockchain philosophy， which is supposed to solve the data sharing， value circulation and business collaboration problems among different blockchain systems. Firstly， a relay mode based cross?chain structure was proposed to provide interoperation services for heterogeneous blockchain systems. Secondly， the detailed design of the relay alliance chain was presented as well as the rules for the participated blockchain systems and their users. Then， the basic types of cross?chain interactions were summarized， and a process for implementing cross?chain interoperability based on smart contracts were designed. Finally， through multiple experiments， the feasibility of the cross?chain scheme was validated， the performance of the cross?chain system was evaluated， and the security of the whole cross?chain network was analyzed. Simulation results and security analysis prove that the proposed channel allocation strategy and block?out right allocation scheme of the proposed scheme are practically feasible， the throughput of the proposed shceme can reach up to 758 TPS （Transactions Per Second） when asset transactions are involved， and up to 960 TPS when asset transactions are not involved； the proposed scheme has high?level security and coarse? and fine?grained privacy protection mechanism. The proposed federated?autonomy?based cross?chain scheme for blockchain can provide secure and efficient cross?chain services， which is suitable for most of the current cross?chain scenarios.