The grating imaging system has the advantages of high resolution, high sensitivity and strong anti-interference ability, and has been widely used in machine vision, biomedicine and imaging spectroscopy and other fields. The coherence and polarization properties of the light field have different effects on the grating imaging system. This paper studies the polarization system of the grating illuminated by partially coherent light based on the unified theory of polarization and coherence. The experiment is carried out using a sinusoidal amplitude grating of 20 lines per mm. The experimental results show that when the coherence is fixed, as the normalized intrinsic frequency of the grating increases, the second harmonic disappears, leaving only the direct current (DC) component and the first harmonic component, which is consistent with the theoretical result. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.