The invention relates to a method for preparing biochar by taking organic solid wastes of agriculture and forestry as raw materials in a wet thermal cracking manner. The method comprises the following steps: cleaning, airing, crushing and screening raw materials, and then mixing and stirring the raw materials with 0.20mol/L zinc chloride solution according to the ratio of 1 to 10, and filtering the mixture after standing for 24 hours, wherein the moisture content of the raw materials is controlled between 45.23% and 69.65%; then putting the raw materials into a tubular atmosphere furnace, and carrying out heating cracking by taking nitrogen of which the flow rate is 0.1m<3>/h as a protective gas, wherein at the first stage, quick heating to 200 DEG C, and then heat preservation for 10 minutes are performed, and the heating rate is 50 DEG C/min; the second stage is a slow heating process, the heating rate is 5 DEG C/min, and heating to 350 DEG C and heat preservation for an hour are performed; the third stage is a slow heating stage, the heating rate is 1 DEG C/min, and heat preservation is performed for 2 hours after heating to 400 DEG C; finally screening the solid substances by a 200-mesh sieve; rinsing and baking into constant weight, so as to obtain a biochar product. The method is wide in raw material source, simple in process, and high in biochar yield; the produced biochar has excellent adsorptive property; the production additional value can be increased by tar, smoke and the like generated in the preparation process; the production cost is reduced.