The invention discloses a method for preparing cellulose aerogel through freeze drying. The method comprises the following steps that (1) cellulose gel D is prepared by using microcrystalline cellulose, lithium chloride and DMAc as raw materials; then, the cellulose gel D is soaked into deionized water for replacement; cellulose gel E is obtained; through purity treatment, cellulose hydrogel F is obtained; (2) after the temperature of the cellulose hydrogel F is lowered to 2 to 4 DEG C, standing is performed; then, the temperature of the cellulose hydrogel is slowly lowered to -6 DEG C from 2 to 4 DEG C; cellulose supercooling hydrogel G is obtained; (3) instantaneous ultrasound disturbance is applied on the periphery of the cellulose supercooling hydrogel G by an ultrasound device; cellulose icy gel H is obtained; through vacuum freeze drying, the cellulose aerogel is obtained. The method has the advantages that at the prefreezing stage of the cellulose hydrogel, the three-dimensional net structure of the cellulose hydrogel can be protected from being damaged due to the influence of ice crystal growth; the problem of cellulose macroporous foams caused by cellulose hydrogel freeze drying is solved.