The invention discloses a method for preparing an auxiliary material of traditional-Chinese-medicine decoction pieces by using microorganisms to ferment straws. The method comprises the following steps: (1) grinding straws into powdery particles with the size of 0.2mm-0.005mm; (2) adding the powdery straw particles, skimmed milk powder, sucrose and water according to the weight ratio of 10:1:1:20-25 into a reaction container, mixing uniformly, sterilizing, then adding mixed strains at a constant temperature of 35-38 DEG C to ferment for 3-5 days; (3) after filtration of fermenting liquid, washing with flowing water, and then drying to obtain powdery pretreated particles; adding the powdery pretreated particles into an oxidation and reduction system with vinyl monomers, and grafting the vinyl monomers onto the powdery pretreated particles to obtain modified cellulose. The method disclosed by the invention has the advantages that the operation is simple, the cost is low, the modified cellulose can be effectively used as the auxiliary material of the traditional-Chinese-medicine decoction pieces, and due to good taste, the modified cellulose can replace the starch to be used as a main food material and the like.