To obtain the size of the pinhole in the spatial filter of the CPA system, Fourier transform and laser multimode theories are applied. And by means of reasonably mathematical approximation and numerical integration, the radial spatial frequency spectrum and the spatial close frequencies of the basic transverse mode and main low-order transverse modes are calculated. The feasibility of applying the close frequency of the basic transverse mode to determining the close frequency of the spatial filter is also discussed, thus the diameter of the pinhole is determined and a widely significant result is achieved. In conclusion, the pinhole, with the optimal diameter of about 6.93 times the diffraction limit length, can fairly filter the high frequencies of most transverse modes except for the TEM00 mode, so that the beam quality is ameliorated and this result is well verified in experiments. Besides, a beam quality-improving factor called “B” based on M2 factor to help describe the performance of the pinhole-lens spatial filter is introduced.