The invention discloses a method for preparing a modified electrode by an electrodeposition chitosan-dye-enzyme composite membrane, which puts a negative electrode into a mixed acid solution containing chitosan, a dye electron transfer medium body and peroxidase and electrifies the mixed solution for electrodeposition, and the chitosan, and the dye electron transfer medium body and the enzyme adhere to the surface of the negative electrode so as to obtain the modified electrode. Utilizing the method of the invention to prepare the modified electrode has advantages of convenience and rapidness, controllable composite membrane thickness, fixed amount of dye electron transfer medium body and fixed amount of enzyme. The prepared reagent-free electrode has advantages of fast response, high sensitivity, good stability and convenient usage.