The invention relates to polluted site repair technology and environmental protection technology, in particular to a grouting repairing additive of a heavy metal pollution site and a preparation method of the grouting repairing additive. The repairing additive comprises powder premix and field mixing liquid. The mass ratio of powder premix to field mixing liquid is (3:7)-(4:6). The powder premix comprises the following materials in mass percent: 15 to 40 percent of biomass charcoal active powder, 10 to 40 percent of modified sea-foam stone powder, 10 to 45 percent of bentonite powder, and 10 to 25 percent of rice hull powder; the field mixing liquid adopts ammonium bicarbonate aqueous solution with the mass concentration of 10 to 20 percent. The preparation method comprises the following steps: uniformly mixing the biomass charcoal active powder, the rice hull powder, the modified sea-foam stone powder and the bentonite powder according to proportion, so as to obtain the powder premix, and uniformly mixing and stirring the powder premix with the field mixing liquid during application, so as to obtain the repairing additive. The repairing additive is low in cost and convenient for construction.