In this paper, using the generalized Huygens-Fresnel diffraction integral and statistical-optics method, the properties of laser beams with amplitude modulation and phase fluctuations propagating through a spatial filter have been studied in detail. The condition for the optimum pinhole size of the spatial filter has been analyzed. Numerical calculations have shown the dependence of the optimum pinhole size on the Fresnel number of the system, the truncation parameter and the phase fluctuation parameter.