The invention discloses a preparation method of a heavy metal curing agent. Red mud is dried, then ground into powder and sieved, a ferrous salt solution, a papermaking black liquor and glyoxal are sequentially added, and red mud composite particles are obtained by microwave activation; shells and diatomaceous earth are washed thoroughly, dried and grounded into powder respectively, and the powderis calcined at a high temperature after being sieved; the red mud composite particles, the shell powder, the diatomaceous earth powder, a phosphate solution and thiamine are blended at a normal temperature, uniformly mixed, dried and then granulated, and the heavy metal curing agent is prepared. The preparation method has the advantages that the preparation process is simple, the source of the raw materials is wide, and the purpose of treating the red mud, the papermaking black liquor and the discarded shells is achieved; the curing agent has good mechanical strength, the ion exchange state content of heavy metal in soil can be effectively reduced, composite heavy metal ions are cured, favorable conditions are provided for the soil ecological environment through the release of trace elements contained in the shells and organic matter in the papermaking black liquor, and a feasible scheme is provided for the redevelopment of waste and the treatment of the heavy metal composite pollution soil.