A system for cogenerating gas-steam based on gasification and methanation of biomass, the system including a gasification unit, a shift unit, a purification unit, a methanation unit, and a methane concentration unit. A waste heat boiler is provided in an upper part of a gasifier of the gasification unit. The methanation unit includes a first primary methanation reactor, a second primary methanation reactor, a first secondary methanation reactor, and a second secondary methanation reactor connected in series. An outlet of the second primary methanation reactor is provided with two bypasses, one of which is connected to an inlet of the first primary methanation reactor, the other of which is connected to the first secondary methanation reactor. The second secondary methanation reactor is connected to the methane concentration unit.