The micro subsurface cracks of fused silica etched with HF solution were recorded using atomic force microscope (AFM) and optical microscope. The damage characteristics of cracks with different structures were studied. Experimental results show that after HF etching, the cracks cross section turns into cosine pattern, and its laser induced damage threshold (LIDT) is not determined by the individual breadth or depth parameters, but by the breadth depth ratio of the crack cross section. The curve of LIDT versus the cracks breadth depth ratio was experimentally obtained. Finite difference time domain algorithm was used to calculate the electric field intensity distribution in the vicinity of a crack with cosine cross section, and the theoretical results agree well with the experimental ones.