Abstract: In large aperture planar optics fabrication, the key process, large precision annular lapping, is difficult to control. The fabricating principle and essence of annular lapping were discussed in this paper. The disadvantages of conventional controlling method were figured out and a real-time off-line monitoring method was proposed. The new method features simple structure, fast measuring and judgment, and can response directly to the change of surface figures. Good results of real-time controlling and surface converging were obtained through experimental validation.