The invention relates to a water purifying agent for pond culture. The water purifying agent is powdery mixture which is prepared by uniformly mixing the following components in part by weight: 100 to 200 parts of bacillus subtilis, 100 to 200 parts of nitrobacteria, 150 to 200 parts of photosynthetic bacteria and 400 to 650 parts of zeolite powder, wherein the water purifying agent preferably comprises 100 parts of bacillus subtilis, 100 parts of nitrobacteria, 150 parts of photosynthetic bacteria and 650 parts of zeolite powder; the bacillus subtilis is light yellow powder and the total count of viable bacteria is more than or equal to 200 hundred million per one gram; the nitrobacteria is light grey powder and the total count of the viable bacteria is more than or equal to 80 hundred million per one gram; the photosynthetic bacteria is grey white powder and the total count of the viable bacteria is more than or equal to 30 to 50 hundred million per one gram; and the zeolite powder is clinoptilolite with particle size of 180 meshes. The water purifying agent has the advantages of improving water quality, conducting the function of oxidation, ammoniation, denitrification, dephosphorization, sulfurization, nitrogen fixation and the like, constructing and maintaining good ecological water environment, preventing and treating diseases, enhancing the anti-disease capacity of animals bred in the pond and improving the survival rate. The water purifying agent has a reasonable formula, is simple to prepare and has obvious application effect.