The invention discloses a soil conditioner capable of reducing activity of heavy metal in soil and a preparation method thereof. The soil conditioner is prepared from sulfydryl modified plant straw powder, an inorganic porous material, caustic dross, humic acid mineral powder, a compound microbial fertilizer and a heavy metal chelating agent. According to parts by weight of the sulfydryl modifiedplant straw powder, the inorganic porous material, the caustic dross, the humic acid mineral powder, the compound microbial fertilizer and the heavy metal chelating agent, the sulfydryl modified plantstraw powder is 3 to 7 parts, the inorganic porous material is 15 to 45 parts, the caustic dross is 1 to 5 parts, the humic acid mineral powder is 20 to 40 parts, the compound microbial fertilizer is2 to 8 parts, and the heavy metal chelating agent is 0.5 to 3 parts. The soil conditioner disclosed by the invention can effectively reduce activity of the heavy metal in acidic heavy metal contaminated soil and further has the advantages of low cost, easiness in implementation, greenness and environmental friendliness.