In high power laser system the light intensity distribution of surface reflection produced by diffractive optical element (DOE) used in beam smoothing was calculated. Calculation showed that with the increasing of transmission distance, the reflected light due to interference would produce a certain number of extreme points, which could be harm to the front element. As a result, in the high-power laser system, it is necessary to place DOE with a tilt angle, so that the reflection light can deviate from the front element. The impact on beam smoothing with changing angle was calculated, and the sharp deterioration of smoothing was found and the corresponding qualitative results were observed in experiment. Phase compensation method was presented, to maintain the ability of the beam smoothing with DOE tilted, and the numerical simulation of phase compensation was given.