In order to understand the transmission phenomena of sub-wavelength metal gratings and the effect of grating parameters on polarization transmission, the polarization transmission is analyzed qualitatively based on the equivalent medium theory. The influence of the grating parameters on polarization transmission and the reason why these influences generated are presented. The mechanism of the transmission peaks is analyzed quantitatively by using the Fabry-Pérot resonance theory. The accuracy of the two theories is carefully verified and analyzed. The qualitative and quantitative analysis of the transmission mechanism of sub-wavelength metal gratings will be helpful for better understanding of the effect of grating parameters on polarization transmission.