The invention discloses a device for synthesizing methane by utilizing renewable energy sources to drive carbon capture and hydrogen generation by water electrolysis. The device is mainly composed ofa renewable energy source power generation system, a hydrogen generation by water electrolysis system, an air carbon capture system and a methanation synthesis system; and the integration manner is asfollows: a hydrogen gas generated by water electrolysis and CO2 captured by the carbon capture system are mixed in a certain ratio, the mixed material is passed into a fixed-bed reactor for a methanation synthesis reaction, the generated methane synthesis gas is passed into a natural-gas pipeline or a natural-gas storage box, and the carbon capture system can realize the series-parallel connection of multi-stage CO2 separation according to the requirements of CO2 separation, wherein the sources of power and heat consumption required by an integrated system are all provided by the abandoned electricity generated by renewable energy source power generation. The device provided by the invention utilizes abandoned electricity generated by the renewable energy sources to perform hydrogen generation by water electrolysis and air carbon capture, so that a raw material gas of methanation synthesis is provided, the absorption capacity of renewable energy source power generation is promoted, the resource utilization of CO2 is realized while carbon emission reduction is reduced, the development and large-scale application of the renewable energy source power generation and carbon capture technologies and a technology of synthesizing a natural gas through methanation in China are forcefully promoted.