The invention provides a method for preparing biochar capable of efficiently removing heavy metals consisting of lead and cadmium from mushroom dregs (consisting of 60% of wheat bran, 20% of corncob and 20% of straw) produced by cultivation of edible oyster mushrooms. The method comprises the following steps: (1) naturally drying oyster mushroom dregs in the air, crushing the oyster mushroom dregsand sieving the oyster mushroom dregs with a 20-to-100-mesh sieve for subsequent usage; (2) placing the sieved oyster mushroom dregs obtained in the step (1) in a muffle furnace and carrying out high-temperature pyrolysis at 500 DEG C for 2 h under anaerobic conditions so as to prepare coarse biochar; and (3) grinding the coarse biochar obtained in the step (2) and sieving the ground coarse biochar with a 100-mesh sieve so as to obtain the biochar with high adsorptivity. The biochar prepared in the invention has excellent lead and cadmium removal effect and can maximally remove 98.5% of leadand 96.4% of cadmium in water. The biochar prepared in the invention does not need modification; the preparation method is simple in process; raw materials are easily available and cheap; and the biochar has good market application prospects.