The invention relates to a preparation method of a river and lake mud solidification and stabilization agent, and belongs to the technical field of environment engineering. The preparation method comprises the following steps: preparing production raw materials comprising phosphogypsum, potassium dihydrogen phosphate and over-burnt magnesium oxide; drying and grinding the production raw materials; uniformly mixing the dried and ground production raw materials according to a mss ratio of phosphogypsum to potassium dihydrogen phosphate to over-burnt magnesium oxide of 1:1:2 in order to obtain the river and lake mud solidification and stabilization agent. The agent produced through the preparation method has the advantages of low treatment cost, high mud solidification intensity, good pollutant stabilization effect and low environment risk.