The invention discloses a water purifying agent for aquaculture and a preparation method of the water purifying agent and belongs to the field of water purifying agents. The water purifying agent is prepared from raw materials in parts by weight as follows: 50-60 parts of construction waste, 10-13 parts of fly ash, 6-8 parts of graphene oxide, 4-6 parts of nano-titania, 1-2 parts of a complex microbial agent, 10-12 parts of chitosan, 10-12 parts of carboxymethyl cellulose, 20-26 parts of polymeric aluminum ferric sulphate and 8-12 parts of polyacrylamide. A water purifying agent frame is formed by the construction waste, filled with microcapsules taking chitosan crosslinked carboxymethyl cellulose as a wall material and graphene oxide, nano-titania and the complex microbial agent are a core material, and assisted by the fly ash, polymeric aluminum ferric sulphate and polyacrylamide, reutilization of industrial wastes is realized successfully while the excellent water purification effect is obtained, and the water purifying agent has good social benefits.