NOVELTY - Preparing marsh gas by three-stage anaerobic fermentation of straw comprises drying straw and sludge, smashing straw and sludge, adding straw and sludge into a mixer, adding water and mixing to form mixture, taking the mixture in a first sealed reactor, anaerobic fermenting, then conveying into a second sealed reactor, anaerobic fermenting, then conveying into a third sealed reactor, anaerobic fermenting, extruding, screening and separating the residual mixture, removing the straw, sludge and water mixture, pumping sludge into the mixer and recycling with fresh straw raw material.    USE - The method is useful for preparing marsh gas by three-stage anaerobic fermentation of straw (claimed).    ADVANTAGE - The method produces 400-500 m3 marsh gas/ton of straw raw material, which contains 50-70% methane.    DETAILED DESCRIPTION - Preparing marsh gas by three-stage anaerobic fermentation of straw comprises drying straw and sludge, smashing straw into particle diameter of less than or equal to 5 cm, smashing sludge into particle diameter of less than or equal to 2 cm, adding straw and sludge into a mixer, adding water and mixing to form mixture, where the amount of straw and sludge is 10-30% and water is 70-90%, and the weight ratio of straw and sludge is 1:0.5-2, taking the mixture in a first sealed reactor, anaerobic fermenting at 25-65 degrees C for 1-7 days, then conveying into a second sealed reactor, anaerobic fermenting at 25-65 degrees C for 5-15 days, then conveying into a third sealed reactor, anaerobic fermenting at 25-65 degrees C for 5-15 days, extruding, screening and separating the residual mixture, removing the straw, sludge and water mixture, pumping sludge into the mixer and recycling with fresh straw raw material. The marsh gas produced in first reactor, second reactor and third reactor is conveyed into a gas storage cabinet, and the straw is at least two of wheat straw, corn stalk, rice straw, cotton stalk, soybean straw, peanut straw and peanut seedling.