The invention relates to a soil heavy 
metal passivator and a preparation method thereof and belongs to the technical field of soil improvement. The soil heavy 
metal passivator comprises 
humin and sulfhydrylation 
humin; the preparation method of the soil heavy 
metal passivator comprises that 
humin is extracted from 
peat soil and subjected to sulfhydrylation for obtaining the sulfhydrylation humin, and the preparation method of the soil heavy metal passivator comprises three concrete steps of pre-treating the 
peat soil, extracting humin and carrying out sulfhydrylation on the humin, namely carrying out 
air drying on the 
peat soil, sieving, weighing defined amount of the peat soil and mixing with 
sodium hydroxide solution, placing mixture on a shaking table and vibrating, standing, centrifuging and separating, washing, and carrying out 
air drying, so that crude humin (HM) is obtained; adding a 
mixed solution of 
mercaptoacetic acid, 
acetic anhydride and 
acetic acid, a concentrated sulphuric acid solution and the crude humin in proportion, stirring, cooling, carrying out reaction in a water bath, washing, and carrying out 
air drying, so that the sulfhydrylated humin is obtained. The preparation method of the soil heavy metal passivator is simple, a technology is novel; the prepared soil heavy metal passivator can increase 
passivation rate of Cd in polluted soil and also can be intensively combined with soil particles, migration of the passivator is reduced, and the soil heavy metal passivator is energy-saving and 
environmentally friendly and is applicable to large-scale popularization.