Abstract: The invention discloses a method for the preparation and use of a slow-release iron-based biochar soil heavy metal passivator. The slow-release iron-based biochar soil heavy metal passivator of the present invention is prepared by an one-step method, wherein iron-based biochar, kaolin and a biological starch are mixed into a core material in a specific ratio; an acidic silica sol and a chitosan solution are prepared, under the effects of an alkaline catalyst and an emulsifier, as a chitosan and silica-sol composite material as a coating, and the iron-based biochar is coated with the alkaline coating material, with the core material and the coating material being controlled at a certain volume ratio. The passivator has a wide raw material source, a simple and convenient preparation process, easy industrialized production, and can passivate the heavy metal arsenic and cadmium efficiently and inhibit the absorption and accumulation of arsenic and cadmium.