The thickness of multilayer coatings in extreme ultraviolet lithography (EUVL) projection system is about 300 nm which is much greater than the work wavelength of 13.5 nm. The most energy can not punch the substrate and optical path difference of dozens of wavelength is induced. Then the image degrades. A model named equivalent work surface (EWS) is built up for coating analysis based on the principle of energy conservation. From the point of erengy modulation, the EWS model transforms the complex physical optics into brief and intuitive geometrical optics, getting recognizable data for commercial softwares. Finally with the aid of EWS, a diffraction-limited virtually-coated system is confirmed, proving the significance of the EWS model. The model can be used for the direction of following alignment and multi-reflection system optimization and coating analysis.