The invention provides a broad spectrum heavy metal activity deactivator preparation method. The present invention relates to a soil conditioning agent preparation method, which comprises the following steps: (A) taking humic acid-rich peat, digest compost or a forest soil humus layer, and carrying out screening to provide a spare use; (B) adding the humic acid-rich material to HCl or HNO3 to soak, and stirring 2-3 times; (C) filtering the soaked material with a bamboo drying pad, washing 2-3 times with clear water, and carrying out air drying to a moisture content of less than 5%; (D) sequentially adding ZnSO4.7H2O, sodium silicate and sodium selenite to the material obtained from the step (C), carrying out aging on the mixed material for 2-3 days and uniformly mixing; and (E) adding limestone powder to the material obtained from the step (D), and completely and uniformly mixing to prepare the broad spectrum heavy metal deactivator. With the combination inhibitor prepared by the preparation method, activities of soil heavy metals can be significantly inactivated, the heavy metals can be inhibited from entering the food chain, and risks on human health by the heavy metals can be reduced.