The Separation of Interference Fringes of the Main Diffraction Maxima (SIFMDM) and the second maximum for Cross Segmented Wedge Array (CSWA) focus system were derived, and the change rules of the Small-scale Irradiation Non-uniformity(SSINU) were confirmed by numerical calculations based on the generalized Huygens-Fresnel diffraction integral theory. The two methods of reducing the SSINU, the Deviation of Wedge Angles (DWA) and the Off-focal Laser Radiation on the Target (OFLRT),were proposed.Theoretical study and experimental results show they agree quite well. By both the DWA and the OFLRT, the SIFMDM can be reduced by 6 times and the SSINU is also improved obviously,when the DWAS and off-focal variables are optimized to δpj=20%,ΔZ=2 mm.