Based on the short wavelength, large numical operture, and short focal depth of optical projection lithography, a focusing method is presented based on Moiré fringe and the principle of triangulation. By analyzing the optical properties of photoelastic modulation and Savart plate, combining the intensity modulation mechanism of dual-grating Moiré fringes, the linear relation between focal plane displacement and output light energy is established in a single grating period. After theoretical analysis and simulated test, it shows precision at nanometer scale with the property of non-contacting, high robustness and well time efficiency, meeting the demands of focusing test in projection lithography.