Proof-of-work (PoW) is an economic response to service and resource abuse or service denial attacks. In general, users are required to perform some complex calculations that take appropriate time, and the answers can be quickly checked by the service side. The time, equipment and energy consumed are taken as the guarantee cost to ensure that the services and resources are used by the real demand. Now this technology has become one of the mainstream consensus mechanisms for cryptocurrencies, such as bitcoin.