The invention discloses a method for producing methane from plant straws and activated sludge. The method for producing the methane from the plant straws and activated sludge comprises the following steps: crushing plant straws into aggregates with the length of 2-6mm, and drying; sequentially adding (NH4)2SO4, CaCO3, K2HPO4 and glucose solid into the crushed plant straw aggregates, uniformly stirring, regulating pH of the obtained solution to be 6-8 with alkali liquor with mass fraction of 0.01-0.03%, so that a fermented material is obtained; and adding the activated sludge into the fermented material, wherein addition amount of the activated sludge is 30-50% of the mass of the fermented material, a fermentation temperature is 35-55 DEG C, and gas is collected by utilizing a drainage method. The method for producing the methane from the plant straws and activated sludge has the advantages that water consumption is low in a production process, no waste water is discharged after fermentation, water resource is saved, environmental protection is facilitated, straw digestion time is short, utilization rate is high, gas yield is high, and crusting is not easily generated.