Beam quality control is the most important segment in high power solid state laser facility construction, for the rigid requirement of precise physics experiments and the operation safety of laser facility in high capacity. The 3ω optics system converts the 1ω beams to the third harmonic, focuses them onto the target and is an important segment influencing beam quality. Based on the B-T theory, increase of beam modulation in 3ω optics was derived under the condition of nonlinear propagation. By the medium-high phase plate constructed according to the surface profile of typical KDP crystals, influence of ∑B3ω on beam near-field quality was investigated. The results show that the fastest increasing frequency and the corresponding fastest increasing factor in 3ω optics system are triple of those of 1ω system, and periods of spatial frequency between 0.1 mm and 1 mm increase fastest. The root-mean-error (RMS) of medium-high frequency aberration has to be less than 1.6 nm to acquire ∑B3ω of 1.5 and 3ω near-field contrast below 1.15.