The correct prediction of subsurface damage (SSD) depth in lapping is an important basis for setting processing parameters. According to the lapping features of fixed abrasive, two typical optical hard-brittle materials (MgAl spinel and quartz glass) were selected to establish their respective two-dimensional discrete element models by using the discrete element method (DEM) and to analyze the impact on SSD (crack) depth of optical hard-brittle materials by above processing parameters. After that, an angle polishing method was used to measure the SSD depth of lapped MgAl spinel and quartz glass and to have it verified by experiment. The results show that when applying fixed abrasive for lapping, the abrasive particle size will have a significant impact on the SSDs of optical hard-brittle materials, which means under same lapping processing, the greater the abrasive particle size is, the deeper of SSD and the denser the micro cracks will be. The comparison between experimental results and DEM simulation results indicates that DEM is able to make quick and effective prediction on SSD depth of optical hard-brittle materials and to provide reference and guidance for subsequent lapping and polishing processing.