The invention provides a soil conditioner and a preparation method thereof. The soil conditioner comprises, by weight, 12 to 20 parts of plant ash, 5 to 10 parts of phosphorous slag, 5 to 9 parts of quicklime, 2 to 5 parts of zeolite, 2 to 5 parts of attapulgite, 1 to 3 parts of Yilong seaweed ooze, 2 to 5 parts of Chinese magnoliavine residues, 2 to 5 parts of large-flowered skullcap root residues, 20 to 30 parts of straw, 8 to 13 parts of hickory nut shells, 0.02 to 0.04 part of rotenone, 10 to 15 parts of mangosteen shells, 3 to 5 parts of an EM microbial agent, 7 to 20 parts of urea phosphate, 8 to 14 parts of urea, 10 to 15 parts of biochemical potassium fulvate and 9 to 12 parts of gamma-polyglutamic acid. The invention has the beneficial effects that the soil conditioner has high porosity and has specific adsorption effect on heavy metal ions; the soil conditioner can adjust the pH value of soil, is rich in trace elements and can elemental N, elemental P and elemental K at the same time; and the soil conditioner allows soil to maintain high porosity and to be long in fertility maintenance time and good in water retention performance.