There are inevitable defects in optical elements of the final target system, which bring in wavefront distortion. According to the intense light beam propagation characteristics of the final target focusing system of high power laser, a model of localized phase distortion brought by optical elements is established. In final target system, a plane beam with Gaussian phase distortion is focused by the lens and then passes through fused silica, such as the subsequent splash-proof panels, in which nonlinear growth is experienced. The influences of Gaussian phase distortion, thickness of fused silica and focal length on the near field beam quality of the final target focusing system are investigated in detail. Results demonstrate that when the wavefront distortion is more serious, the thickness of fused silica is longer, and the focal length is shorter, the near field beam quality is worse and the growth of middle-high frequency component is bigger.