With the improvement of lithography resolution node, high precision focus detection method becomes more and more important. The focus detection method which is based on moire fringe phase analysis and the triangulation principle is introduced. By analyzing optical modulation of photoelastic modulator and Savart plate, adjusting the parallel plate, the light intensity modulation which is caused by focal plane displacement changes in sine or cosine form. By analyzing the experimental data, focusing accuracy of this method is at nanoscale, hard real- time, and non- contact. It can meet the need of projection lithography with the line width of less than 100 nm.