The invention relates to a method for improving the mechanical properties of dyed polyimide fibers. The method comprises the following steps: infiltrating polyamide acid fibers with an acetic anhydride/pyridine solution and carrying out chemical imidization; subjecting the fibers having undergone chemical imidization to drying and thermal cyclization so as to obtain polyimide fibers; pretreating the polyimide fibers via alkaline etching; and carrying out washing and drying, then dyeing the polyimide fibers with a dispersive cationic dye and carrying out washing and drying after dyeing. According to the invention, chemical imidization and thermal imidization are both carried out so as to reduce the defect structures of the fibers, improve uniformity of imidization and enhance the mechanical properties of the fibers; and alkaline etching is carried out so as to realize surface roughening of the polyimide fibers, attenuate total reflection of light, improve diffuse reflection of light and open a part of imide bonds on the surface of the fibers, which is beneficial for ionic bonding between cations on dye molecules and carboxyl groups in subsequent operation, so the problem of difficulty in dyeing of the polyimide fibers is effectively overcome. The method is simple in process; and the dyed fiber has good color depth, color fastness and mechanical properties.