The invention discloses a method for preparing corn polypeptide by taking zein as raw materials, which is designed for solving the defects that the traditional method of lifting alkali but sinking sour is easy to damage the active ingredient of the zein and the product has many impurities by hydrolyzing the yellow corn powder directly. The method is that: dissolving sufficiently the zein powder in alkaline water solution, conducting the limited zymohydrolysis on the zein with alkali protease and then undertaking the rough filter, decolor by using active carbon, bitter removing, fine filter, sterilization, vaccume compression, spraying and drying, thus achieving the corn polypeptide. The method selects the raw material with high content of proteide but without impurities as amylum, fibre and coloring matter, etc. and takes the alkaline water solution as the latent solvent which enables the zein dissolved sufficiently and facilitates the combination of the kali protease and the zein typsin and also enables the corn polypeptide product contain alkali ion which is good for human body. The corn polypeptide has the advantages of simple technics, easy controlled zymohydrolysis degree and good quality and effects.