The invention discloses an anaerobic reactor for mixed fermentation of pig manure and straw to produce marsh gas, and belongs to the technical field of environmental protection. Straw having the solidcontent of 80-95% and obtained after crushing pretreatment is mixed and stirred with pig manure having the solid content of 5-20% to form a raw material having the solid content of 10-40%, and a traditional low-concentration fermentation mode is changed. A three-stage screw conveyor is used for feeding materials into the anaerobic reactor, fully mixed homogeneous stirring with triple stirring isadopted for replacing traditional top stirring or side pushing flow stirring. The conversion efficiency of marsh gas and the operation stability of the reactor are improved. The mixed fermentation ofthe pig manure and the straw is more conducive to balancing the carbon-nitrogen ratio of anaerobic reaction and promoting the stability of the fermentation system. Under stable operation conditions, the marsh gas yield is higher than 450 m<3>/kg VSS and is about 15% higher than that of traditional wet fermentation.