The invention provides a method for preparing porous boron-carbon-nitrogen nanosheets through freeze drying. The method includes the following steps that 1, a boron source, a carbon source and a nitrogen source are mixed in proportion, a raw material mixture is formed, water is added to dissolve the raw material mixture, and a raw material mixture solution is formed; 2, the raw material mixture solution is condensed and cured and then subjected to freeze drying; 3, powder obtained after freeze drying reacts for 0.5-12 h at the temperature of 800-1,200 DEG C in the atmosphere protection or vacuum environment; 4, the powder is ball-milled in a ball mill for 2-8 h; 5, after the ball-milled product is cooled to room temperature, the product is cleaned and dried, and the porous boron-carbon-nitrogen nanosheets are obtained. The method is simple in operation procedure, the yield can be increased, and the cost can be reduced. Various parameters in the reaction process are easy to monitor and control, the reaction mechanism can be easily studied, most critical influence factors are found out, and process conditions are stabilized as soon as possible; environmental protection is low, and environmental protection is promoted.