The unavoidable defects existing in optical components will influence the beam propagation in the optical system. Based on the Fresnel diffraction integral and the definition of angular spectrum, the analytical expressions for the light-intensity distribution and angular spectrum of a Gaussian beam modulated by finite-amplitude modulated defects are derived. The influence of the size and the modulation amplitude of the defects on the intensity distribution and angular spectrum of the beam is also studied. It is shown that the intensity distribution will recover to Gaussian distribution after propagating a certain distance. The larger size of defects is, the longer propagation distance is for the beam recovering to Gaussian distribution. Furthermore, the angular spectrum of the modulated beam in the low-frequency area becomes smaller and the angular spectrum in the high frequency area becomes greater with the increase of the size and the modulation amplitude of the defects.