The work aims to directly measure the subsurface damage of the workpiece after machining, observe the whole morphology and expansion of subsurface crack and explore the morphology and depth of the workpiece subsurface damage under different loads, so as to guide the design of the machining process and reduce the subsurface damage of the workpiece after machining. Static indentation test was used to generate indentation on quartz glass surface. Side layer-by-layer polishing and etching method was adopted to investigate the effect of load on subsurface cracks of the workpiece and the crack extension on the subsurface. The relationship between the indentation load and subsurface crack depth was studied through the regression analysis method. When the load was less than 0.098 N, the indentation on the workpiece was in a regular diamond shape. There was plastic deformation on the surface, but no crack on the subsurface. When the load was greater than 0.245 N, there was a noticeable bulge on the surface of the workpiece. The subsurface cracks appeared and the crack depth increased as the load increased, and the crack morphology gradually changed from “splayed” radial cracks to median cracks and plurality of radial cracks. The indentation load is related to the depth of subsurface cracks as a power function. When the load is small, there is a certain plastic processing domain for the workpiece. When the load is greater than a certain value, the crack begins to gradually appear on the subsurface of the workpiece. The bigger the load is, the more likely the surface would be bulged, collapsed, peeled off, etc. The subsurface crack is also deeper.