The invention discloses a preparing method for a microbial preparation for reducing nitrite in aquaculture water, and belongs to the technical field of microbial preparations. The preparing method includes the steps that grease in small intestines of a pig are taken and sealed for natural smelliness, the smelly grease and smelly crucian carp are mixed and stewed into paste, soybean nodulation powder is added after the grease and crucian carp mixture mildew naturally, after fungi grow to certain length, distilled water is added and mixed, supernate is obtained through filtering and centrifugally separated in a high-speed centrifuge, and supernate is collected again to obtain the microbial preparation. The microbial preparation has the advantages of being rich in organic matter and nitrosomas and capable of effectively promoting conversion of nitrite in the water into nitrogen to be exhausted from the water so as to reduce the content of the nitrite in the water. The preparing method is simple in preparing process, and the obtained microbial preparation is good in using effect and enables the content of nitrite in sludge of the aquaculture water to be reduced by 80-90% than the original content.