Rotating paraboloid model is establishd, and three-dimensional finite-difference time-domain method is used to simulate pit-shaped cracks on fused silica rear-surface. The light intensification with its depth, width, gap distance and etch value are investigated under 355 nm laser incident. Results show that the strongest modulation is located at the connection area between pit and pit, and the modulation become strong with approaching to the surface. The maximum light intensity enhancement factor (LIEF) is 11.53 when the breadth depth ratio ranges from 2.0 to 3.5 and gap distance close to 1/2 width. As gap distance greater than the width, the modulation reduces greatly, which is equal to a single pit. For 60-width, 30-depth and 30-gap distance cracks, the maximum LIEF is 11.0 during the acid etching. As the gap distance is less than 300 nm, the diffraction of the light field makes the neighbor pits connective.