Various chains have been created since the development of blockchain. Digital assets on different chains are isolated from each other，which performs “value island”. Cross-chain targets such issue and it aims to eliminate “value island” by bridging these islands. How to connects and exchanges value between different blockchain ledgers in adistributed manner，will be an important point in the development of the blockchain industry. Cross-chain technology for blockchain，just like TCP/IP for Internet. Basic Communication Protocol-TCP/IP connects the isolated systems and now we have today’s global Internet. Cross-chain mechanism will bring a key breakthrough to the blockchain. The cross-chain technology has been discussed by many blockchain projects. However，what is the essence of cross-chain and the problems cross-chain can solve are rarely addressed. Similarly，various teams，such as Cosmos and Polkadot，have recently offered blockchain connectivity solutions for networks running on various protocols，regardless of their consensus mechanism. This paper hopes to help us understand the cross-chain technology by discussing the perspective of cross-chain，cross-chain attributes，value significance，and technical key difficulties. At the same time，cross-chain technology by Wanchain will be introduced and explained.