To reduce the effect of manufacturing error of mold inserts on the optical performance of molding diffractive optical elements (DOEs), single point diamond turning process of microcrystalline aluminum alloy mold inserts with a curved substrate diffractive microstructure surface is studied. Based on the shadowing effect and scattering effect, the influence of mold insert manufacturing error on the diffraction efficiency of DOEs is analyzed, a mathematical relationship model among the tool radius, tool deflection angle, and diffraction efficiency is established, and an optimization method for improving machining accuracy is proposed. The turning experiment of diffractive optical mold inserts is carried out under the guidance of the method, and the expected diffraction efficiency is compared with the experimental results. The results show that using process parameters obtained by the proposed method can reduce the influence of mold insert manufacturing error on diffraction efficiency and improve the optical performance of DOEs, which provides a reference for SPDT diffraction optical mold insert manufacturing.