In order to manufacture high-precision plastic diffraction microlenses, this paper proposes to select the process parameters in a wide range by Taguchi method, determine the process parameters that significantly affect the molding precision of microstructure through the signal-to-noise ratio, and obtain the optimal parameter combination through multi-objective optimization by weighted comprehensive scoring method. The results show that the holding pressure, mold temperature, and holding time have a significant impact on the microstructure molding precision. In order to improve the precision of diffraction microstructure, the main source of diffraction microlens injection molding error is analyzed and the error compensation model is established. The experimental results show that the diffraction microstructure of the injection molding has a height error of 5.69% and a period error of 6.16%, and the injection molding precision of the diffraction microstructure is significantly improved.