The invention discloses a soil remediation agent. The soil remediation agent comprises, by weight, 8-12 parts of hydrated lime, 30-50 parts of limestone, 40-60 parts of bentonite, 5-7 parts of sepiolite, 5-10 parts of biological charcoal, 2-3 parts of diethylenetriamine pentaacetic acid, 2-3 parts of sodium dimercaptosulphonate, 3-5 parts of sodium lignosulfonate, 1-3 parts of cysteine, 1-1.8 parts of sodium potassium tartrate tetrahydrate, 1-1.5 parts of phosphate ore and 0-0.5 part of sodium sulfide. A preparation method of the soil remediation agent includes the steps of firstly, weighing the phosphate ore, the sepiolite and the biological charcoal, and grinding into powder; secondly, adding the cysteine, the sodium potassium tartrate tetrahydrate and the sodium sulfide into the ground powder, and stirring; thirdly, continuously adding the rest of substances, and mixing. The preparation method is simple and low in cost, and the prepared soil remediation agent can lower the heavy metal activity in soil, improve the soil, increase soil fertility and increase crop yield.