NOVELTY - A catalytic methanation of synthesis gas comprises (A) converting with fluidized bed and (B) converting with fixed bed. The step (A) includes introducing synthesis gas into fluidized bed reactor and catalyzing for methanation to obtain intermediate product; and treating intermediate product via gas-liquid separation for obtaining gas intermediate product. The step (B) includes obtained introducing gas intermediate product into fixed bed reactor for catalyzing remained synthesis gas for obtaining methane.    USE - Method is used for catalytic methanation of synthesis gas (claimed).    ADVANTAGE - The conversion rate of raw material synthesis gas can be more than 98%. The technical flow of the method is simplified, the space time yield of methane is improved, and industrial application value is much better as compared with conventional method.    DETAILED DESCRIPTION - A catalytic methanation of synthesis gas comprises (A) converting with fluidized bed and (B) converting with fixed bed. The step (A) includes introducing synthesis gas into fluidized bed reactor and catalyzing for methanation to obtain intermediate product; treating intermediate product via gas-liquid separation for obtaining gas intermediate product; and discharging heat via heat exchange medium of heat exchanger in fluidized bed reactor, and overheating side product for converting into steam. The step (B) includes obtained introducing gas intermediate product into fixed bed reactor for catalyzing remained synthesis gas for obtaining methane. An INDEPENDENT CLAIM is included for a device for carrying above method which comprises fluidized bed reactor (3) for catalyzing synthesis gas for methanation; heat exchanging device (4) placed in fluidized bed for removing heat discharged from methanation; separating device placed in inner or outer part of fluidized bed for separating intermediate product and catalyst; and fixed bed reactor (9) connected with separating device for catalyzing remained synthesis gas for methanation.    DESCRIPTION OF DRAWING(S) - The drawing is schematic diagram illustrating a device for catalytic methanation of synthesis gas.    Heat exchanging device (1, 8)    Fluidized bed reactor (3)    Heat exchanging device (4)    Fixed bed reactor (9)    intermediate groove device (10)