The invention discloses a method for promoting zizania aquatica straw decomposition. The method comprises the following steps: selecting a composting place; preparing a fermentation liquor, wherein brown sugar is dissolved in water at the temperature of 80-100 DEG C, the ratio of the brown sugar to the water is 1: (7-9), an obtained mixture is cooled to 30-40 DEG C, then an EM microbial agent is added, sealed fermentation is conducted for 7-12 days, and the fermentation liquor is obtained; preparing an organic fertilizer, wherein zizania latifolia straws are crushed and then are divided into aplurality of parts, mixing each part of the crushed zizania latifolia straws with livestock and poultry manure and the fermentation liquor respectively to obtain mixed products, the carbon-nitrogen ratio of the mixed products is adjusted to be (20-30): 1 with urea, the plurality of the mixed prdoucts are mixed, the water content is adjusted to be 60-70%, long strips are obtained through stacking,and are tightly coverd with plastic cloth, when the composting center reaches 60 DEG C overturning is carried out, and anaerobic fermentation is carried out for 40-55 days to obtain a decomposition product. The method has the advantages of being high in decomposition speed, high in decomposition degree, convenient to operate and easy to master by farmers.