The invention relates to a process for synthesizing methane through synthesis gas. The process comprises the following steps of: according to the weight ratio of slurry bubble column methanation catalyst to solvent being 10-30:100, feeding the slurry bubble column methanation catalyst into a slurry bubble column reactor, taking the synthesis gas as a raw material, carrying out methanation reaction, and forming slurry bubble column outlet gas through methane produced through reaction, unconverted CO, H2, a little dimethyl ether produced through reaction, CO2 produced through water gas shift reaction, and the like; and isobarically and isothermally feeding the slurry bubble column outlet gas into a fixed bed reactor filled with fixed bed methanation catalyst, carrying out methanation reaction again on unconverted H2, CO, the CO2 produced through water gas shift reaction, and the like, splitting the little dimethyl ether to generate methane, and separating fixed bed outlet gas to obtain the product gas in a condensation way. The process has the advantages of high CO conversion rate, and high methane selectivity.