NOVELTY - Synthesis of methane comprises (A) catalyst reduction and (B) synthetic reaction. The step (B) comprises providing slurry bed methanation catalyst and solvent at mass ratio of 10-30:100, loading slurry-bed methanation catalyst into slurry bed reactor, using synthesis gas comprising hydrogen gas and carbon monoxide, and processing methanation reaction at 0.5-3.0 MPa, 270-380 degrees C and 5000-30000 mL/g-hour. A slurry bed exit gas is formed from methane produced by reaction and unconverted carbon monoxide, hydrogen gas, dimethyl ether and carbon dioxide produced by water gas shift reaction.    USE - Method for synthesizing of methane (claimed).    ADVANTAGE - The method has high percent conversion of carbon monoxide and high selectivity of methane.    DETAILED DESCRIPTION - Synthesis of methane comprises (A) catalyst reduction and (B) synthetic reaction. The step (A) comprises:    (A) crushing slurry bed methanation catalyst to 200 meshes, and using diluted hydrogen gas and putting in fluidized bed pre-reducer, where ratio of hydrogen gas to argon is 5-10:90-95, gas airspeed is 200-1000 mL/g-hour, reduction pressure of 0.3-1MPa, reduction temperature of 400-500 degrees C, reduction heating rate of 2-8 degrees C/minute, and reducing for 10-20 hours; and    (B) crushing fixed bed methanation catalyst to 200 meshes, in fluidized bed pre-reducer, and using diluted hydrogen gas and putting in fluidized bed pre-reducer, where ratio of hydrogen gas to argon is 5-10:90-95, gas airspeed is 200-1000 mL/g-hour, reduction pressure of 0.3-1MPa, reduction temperature of 400-500 degrees C, reduction heating rate of 2-8 degrees C/minute, and reducing for 10-20 hours.    The step (B) comprises:    (A) providing slurry bed methanation catalyst and solvent at mass ratio of 10-30:100, loading slurry-bed methanation catalyst into slurry bed reactor, using synthesis gas at mole ratio of hydrogen gas and carbon monoxide at 2.5-3.5:1 as raw material, and processing methanation reaction at 0.5-3.0 MPa, 270-380 degrees C and 5000-30000 mL/g-hour;    (B) under condition of constant pressure and temperature, loading slurry bed exit gas into fixed bed reactor containing fixed bed methanation catalyst; and    (C) under condition of reaction temperature 350-500 degrees C, using slurry bed reactor raw materials as standard and gas airspeed 5000-30000 mL/g-hour and once again processing methanation reaction on unconverted carbon monoxide, hydrogen gas and carbon dioxide produced by water gas shift reaction pyrolyzing small amount of dimethyl ether and obtaining methane, and processing condensation and separation on fixed bed exit gas and obtaining product gas.    A slurry bed exit gas is formed form methane produced by reaction and unconverted carbon monoxide, hydrogen gas, small amount of dimethyl ether and carbon dioxide produced by water gas shift reaction.