Potassium dihydrogen phosphate (KDP) crystal is widely used in navigates spaceflight, national defenses, energy sources and information technology fields based on its excellent nonlinear optical property. Surface quality of KDP crystal influences the property and life time of the device directly. So detection and analysis of the damage induced during machining process, especially on subsurface, should be solved. In this paper, surface damage of the KDP crystal, which machined from #600 grinding wheel with different feed rate, was detected by optical microscope. Cross section and selective etching were used to analyzing the subsurface damage. Because #600 grinding wheel mainly used in coarse grinding and the removal rate is high, the results shown that there was obvious scratch, crack and crushing on the machining surface. When the feed rate is 10+m and 40+m, the subsurface damage depth is 7.41+m and 8.96+m corresponding. This study is a kind of guide for following precision grinding, polishing machining time and removal amount.