Abstract: Aiming at the phase measurement deflectormetry(PMD), it is difficult to control the low-order error of surface figure in optical elements surface figure detection with high accuracy. The basic principle of the optical flat elements surface figure detection by PMD was introduced. The research progress of improved surface figure reconstruction algorithm and systematic error deduction method of relative detection and four-step shearing of PMD technology was described. Based on PMD technology, the splicing detection of flat glass with the size of 398.7 mm×422.8 mm and the elimination method of parasitic reflection influence existed in the flat elements were analyzed. The RMS of detection accuracy of established 6-camera slope splicing detection system could reach to 1 µm. Multi-frequency fringe method and binary fringe method could be effectively eliminated the influence of parasitic reflection, which provided a feasible scheme for the surface figure detection with high accuracy of front and rear surfaces of the large-caliber optical flat elements.