The invention discloses a preparation method and applications of an aquatic product sewage treatment agent. The preparation method comprises following steps: mussel shell is treated with a diluted acid solution, and then is subjected to high temperature treatment and grinding so as to obtain mussel shell powder; an organic modifying agent and aluminium chloride are dissolved with deionized water,roasted bentonite is added, and stirring, reaction, and grinding are carried out so as to obtain modified bentonite; the mussel shell powder, sodium alginate, polyferric chloride, polymeric aluminium,the modified bentonite, polyacrylamide, vegetable fiber, and ethylene glycol-bis-(2-aminoethylether)-N,N, N', N'-tetraacetic acid (EGTA) are uniformly mixed, are subjected to pelletizing moulding ina pelletizer, and are dried so as to obtain the aquatic product sewage treatment agent. The beneficial effects are that: the advantages of the ingredients are achieved, synergetic effects are achieved, nitrogen, phosphor, and organic pollutants can be removed effectively, the aquatic product sewage treatment agent can be recycled, and can be used for aquatic product sewage treatment, the treatmenttechnology is simple, the operationality is high, efficiency is high, and no side effect on operation personnel is caused .