The Computer Controlled Active Lap (CCAL)’s real-time surface shape in fabricating large off-axis aspheric mirror is analyzed by using coordinates transform theory, and a universal method for calculating the lap’s real-time profile is also presented. The physical parameters of off-axis aspheric mirror in New Solar Telescope (NST) is used to be simulated as an example, and the active lap’s surface shape in grinding and polishing process is got. The application in two aspheric reflected mirrors (Φ1 030 F/1.6 ellipsoid and Φ1 250 F/1.5 paraboloid)’ grinding shows that this method can be used to describe the lap’s surface shape exactly in fabrication. The results from analysis and calculation can be used in next off-axis aspheric mirror fabricating.