With the development of blockchain technology, various blockchains with different characteristics and applications such as Bitcoin, Ethereum, other public chains, as well as private and consortium chains coexist in large numbers. Due to the independence of blockchains, the data communication and value transfer between existing blockchains are still facing challenges, and the problem of value isolation gradually emerges. The cross-chain technology is an important technical means for blockchains to realize interoperability and enhance scalability. This article systematically summarizes the achievements in the cross-chain technology field. Firstly, the requirements of cross-chain technologies and the technical difficulties they face are analyzed. Secondly, the cross-chain technologies those are under development are summarized and the mechanisms and implementations of 24 mainstream cross-chain technologies are introduced. Then, the security risks of cross-chain technology are analyzed, and 12 major issues are listed. Finally, the trend of future development of cross-chain technology is summarized and discussed.