The invention relates to a soil conditioner and a preparation method thereof. The soil conditioner is prepared from raw materials in parts by weight as follows: 2-10 parts of biochar, 1-5 parts of wood vinegar, 5-10 parts of compound microorganism bacteria and 3-5 parts of an adhesive. The soil conditioner can keep the soil fertility under the adsorption function of the biochar, adsorb nitrogen, phosphorus and potassium in soil, promote activity of microorganisms in the soil to promote decomposition of the soil, increase the porosity of the soil, reduce the soil bulk density, increase the gas permeability of the soil, eliminate the conditions of fertility declination and poor physical and chemical properties of soil and the like, effectively eliminates the phenomena of soil hardening, acidification and the like, improves the quality of agricultural products, promotes early maturing of crops and increases the crop yield.