Based on nonlinear propagation of laser and frequency conversion theory, the effects of the relationship between 1ω phase and 3ω phase on 3ω near filed were studied. The results show that the B-integral corresponding to the sharp increase of 3ω near field contrast was far less than that of 1ω, even the B-integral of 1ω is less than 1.8 rad, 3ω near field occurs filament. This explained the unusual damage in final optics assembly(FOA) in prototype experiments of SGIII. The results plays an important role in reducing the risk of UV optics damage in FOA.