The main factor of laser induced damage is the modulation caused by the scratch in the subsurface of the fused silica. The three-dimensional Hertzian conical scratch (HCS) model on the exit surface is establised in this article. Three-dimensional finite-difference time-domain method is used to calculate and simulate the electric field intensity distribution in the vicinity of HCS. The effects of depth, radius and gradient angle of the HCS on incident light field modulation are analyzed, respectively. The results show that the electric field enhancement in the center of HCS is the largest. This area is apt to be damaged. The maximal electric field intensity in the fused silica will be enhanced as the depth of the HCS changes from λ to 19λ/2. The fused silica is liable to be damaged while the radius of HCS is below 15λ. When the radius is larger than 175λ, the maximal electric fields in the fused silica tend to be 2.5 V/m. And the maximal electric fields are independent on the radius. The effect of light intensity enhancement is more obviously when total internal reflection occurs between internal interface of HCS and the rear surface of fused silica.