The invention discloses a method for preparing cellulose from straws by using a biological method, and belongs to the technical field of biochemical industry. The method comprises the following steps of: removing dust in the straws, crushing the straws to be 20 to 40 meshes, adding the crushed straws into a reaction tank, adding water in an amount which is 4 to 5 times weight of the straws, conveying the straws to a high-concentration mill after the straws are soaked for 1 hour, and pulping; and adding the pulp into a reaction tank, adding a composite biocatalyst, mixing uniformly, heating tothe temperature of between 60 and 80 DEG C, and reacting for 1 to 3 hours; and removing impurities to obtain coarse pulp by screening after the pulp is discharged, delivering the coarse pulp to a low-concentration mill, pulping, performing dehydration through a pulp extruder, performing defibering by using a defibering machine, drying in a roller dryer, drying and crushing to obtain the straw cellulose. Because the straws are used as a raw material for preparing the cellulose, the cost is reduced by 50 percent compared with the traditional process for extracting the cellulose; and the biological method has no pollution and zero release, and protects the environment.