A method for cogenerating gas-steam based on gasification and methanation of biomass. The method includes: 1) mixing oxygen and water vapor with biomass, transporting the resulting mixture via a nozzle to a gasifier, gasifying the biomass to yield crude gasified gas, and transporting superheated steam having a pressure of 5-6 MPa resulting from sensible heat recovery to a steam turbine; 2) adjusting the hydrogen/carbon ratio of the crude gasified gas generated from step 1) to 3:1, and eluting the crude gasified gas whereby yield purified syngas; 3) introducing the purified syngas from step 2) to a methanation unit and transporting intermediate pressure superheated steam generated in the methanation unit to the steam turbine; and 4) concentrating methane of synthetic natural gas containing trace nitrogen and water vapor obtained from step 3) through pressure swing adsorption.