A method for producing methane and co-producing a liquid fuel from a carbonaceous material includes the steps: crude synthesis gas obtained after gasification of the carbonaceous material is purified to remove impurities in the crude synthesis gas, and thus purified synthesis gas is obtained; at least a portion of the purified synthesis gas and water are in contact with a mixed catalyst, a product obtained after contact is subjected to gas-liquid separation, and thus a gas product and a liquid product are obtained, wherein the mixed catalyst contains a water-gas shift catalyst and a Fischer-tropsch synthesis catalyst; and a methanation gas raw material is in contact with a methanation catalyst, wherein the methanation gas raw material contains the gas product, and the H2/CO molar ratio in the methanation gas raw material is 2.8-3.5. According to the method, the methane can be produced and the liquid fuel can be co-produced, moreover, the load of water-gas shift in the process is effectively reduced, the energy consumption is saved, the utilization efficiency of carbon is improved, and furthermore, the method is performed directly on a methane production device of the prior art without increasing equipment.