The spatial filter pinhole in high power solid state laser is used to filter the spatial frequency whose nonlinear growth is fast to improve the beam quality that is destroyed by optical elements defect. Thus the diameter of pinhole is the key factor related to the filtering effect. The nonlinear growth in boost amplifier of Sheng Guang(SG)Ⅲ prototype is calculated and the characteristics in different spatial frequency are analyzed. Based on the simulation results, the pinhole diameter design in boost amplifier is discussed. The pinhole closure caused by plasma and the low frequency circle in near field in pinhole diameter design is analyzed. According to the simulation results, the pinhole diameter design principle in boost amplifier of SGⅢ prototype is discussed.