Device and process for preparing natural gas by virtue of double-fluidization bed gasification of biomass

The invention provides a device and a process for preparing natural gas by virtue of double-fluidization bed gasification of a biomass. The device comprises a burning carrier heating unit, a pyrolysis gasification unit, a biological carbon/carrier separation and conveying unit and a methane synthesis and tar returning unit, wherein the pyrolysis gasification unit is connected with the burning carrier heating unit, the biological carbon/carrier separation and conveying unit and the methane synthesis and tar returning unit. The other components are converted into methane gas as much as possible while the biological carbon is obtained, the content of methane in a pyrolysis product is about 10%, the pressure for subsequent synthesis is reduced, the tar in hydrolysis gas is used as a fuel for a double-fluidization bed, and the problems that combustion cannot be stabilized at a low temperature and alkali metals causes scorification in a high-temperature state when other double-fluidization beds adopt biological carbon combustion can be avoided to ensure that the temperature of a combustion furnace can be controlled very flexibly. The device and the process provided by the invention fully use different pyrolysis products of the biomass, can realize treatment of various biomasses, and are wide in raw material application range.