The invention discloses a method for producing xylanase by anaerobic fermentation of corn straws. A fermentation strain used in the method comprises natural coexisting Piromyces Yak TZ+M. ruminantium coculture separated from rumen fluid of yak fully grazed in a Tianzhu Nanniwan pasture on the Tibetan plateau, and the collection number of the coculture at the China General Microbiological Culture Collection Center is CGMCC No. 12952. The coculture serving as the strain is used for anaerobic fermentation of the corn straws, the xylanase can be efficiently produced, the enzymatic activity of the xylanase generated by degrading the corn straws through the coculture within the culture period of 7 days reaches up to 6905mU, and the method is short in fermentation time, simple and convenient in fermentation process and has a wide application and development prospect.