Casein grafted acrylonitrile fibers were treated by different concentration alkali solutions. The fibers component were investigated using chemical analysis before and after alkali treatment. The effects of alkali pH value on homopolymer PAN, free casein, grafted casein and grafting degree of the fibers were discussed individually. At the same time, the mechanical properties of the fibers treated by alkali at various pH values were tested, and the intrinsic and extrinsic factors affecting fibers mechanical properties were analyzed. The results can play a certain reference role to the early manufacture and finishing process of casein grafted acrylonitrile fibers.