An optimized design method for a diffraction optical element plated with an anti-reflection coating is disclosed and belongs to the technical field of optical design. Only effects exerted on diffraction efficiency by a diffraction optical element body is considered via conventional technologies while effects exerted on the diffraction efficiency of the diffraction optical element body by the anti-reflection coating is not considered. According to the optimized design method for the diffraction optical element plated with the anti-reflection coating, surface diffraction microstructure height of the diffraction optical element is first designed. The method is characterized in that later an additional phase introduced via all coating layer material and thickness of a surface anti-reflection coating system of the diffraction optical element is calculated, and finally working height of a surface diffraction microstructure of the diffraction optical element body is obtained after height equivalent to the additional phase of the anti-reflection coating is subtracted from the surface diffraction microstructure height. The method is applied to optimized design of the diffraction optical element plated with the anti-reflection coating, and diffraction efficiency of design wavelength is enabled to reach 100 percent while anti-reflection of the diffraction optical element is realized.