The invention relates to a method for preparing marsh gas by anaerobic fermentation of agricultural straws. The problem that the marsh gas is prepared from agricultural straws with the high yield is effectively solved. The method comprises the following steps: selecting the air-dried agricultural straws, an inoculum, nitrogen, biomass charcoal and water as raw materials, wherein the weight ratio of the agricultural straws to the inoculum is 1 to (0.5-2), the weight ratio of a mixture of the agricultural straws and the inoculum to the water is (10-30) to (70-90), the weight ratio of the biomass charcoal to the agricultural straws (1-10 percent) to (90-99 percent), and the weight ratio of the biomass charcoal to the nitrogen is (20-35) to 1; and firstly loading the biomass charcoal and the nitrogen in a reactor, then adding the agricultural straws, the inoculum and the water, uniformly mixing, and closing for performing the anaerobic fermentation under the conditions that the fermentation temperature is 25-65 DEG C and the fermentation time is not less than 35 days to obtain the high-yield marsh gas. The raw materials used in the method are abundant, the method is simple, and waste is recycled, so that the technical difficulty that the high-yield marsh gas is prepared from the agricultural straws, sludge and biogas residue is effectively solved, and the environmental pollution is greatly reduced.