The invention discloses an anaerobic fermentation method for straw. The anaerobic fermentation method for the straw comprises the following steps that (1) sulfite is adopted to pretreat straw powder; (2) the pretreated straw powder is subjected to ultrasonic enzymolysis; (3) then the straw powder is soaked by using a sodium hydroxide solution; (4) the straw powder and the solution are separated, and acidogenic fermentation is conducted correspondingly; and (5) finally, methane-producing fermentation process is conducted. According to the anaerobic fermentation method for the straw, the yield of metabolite in the acid producing stage can be increased effectively so that the yield of methane and the use ratio of the straw can be increased.