To meet the demand on lightweight manufacturing of PEEK-based polymer composite involute spline couplings, this work proposes a forming and processing method for involute spline couplings of PEEK-based polymer composite materials based on traditional injection molding methods; The spline coupling’s mold design scheme has been adopted with a combination cavity, and the selection of the injection molding machine, the design of the mold frame, as well as the design calculation of the Harvard style slider, have all been completed. Subsequently, MoldFlow software was utilized to conduct a mold flow analysis, which evaluated the rationality of the mold structure design, while the injection molding process parameters were compared and optimized to ensure the quality of the product molding, all in a passive manner. The results show that the optimal injection parameters for the spline couplings were a mold temperature of 175 ℃ and a melt temperature of 375 ℃. Under this parameter combination, the cavity filling of the spline couplings combination is balanced, the injection pressure is stable, and the number of air pockets and fusion lines is significantly reduced. At the same time, the results of the mold flow analysis also verified the rationality and feasibility of the combined cavity mold design scheme, it provides new methods and ideas for the injection molding process of PEEK-based polymer composite involute spline couplings and lays a theoretical foundation for the application of lightweight involute spline couplings.