Surface damages of optical materials are inevitably introduced during machining processes. These damages heavily degrade the working performance and lifetime of optical materials-based production. Compared to the machining process of grinding, the art of polishing smoothly removes the damage layer of the sample; however, it is often difficult to ensure the complete removal of the damage layer. Therefore, to determinethe sample's surface quality during polishing periods is very necessary despite difficulties in the measurement technique.Here, we analyzed the typical surface structure of a polished sample and studied the measurement principle of the quasi-Brewster angle method. Simulations show the advantages of this approach to characterize the surface quality of a polished optical plate. As a demonstration, Nd∶GGG optical plates were polished with different processing time and tested carefully with a spectroscopic ellipsometer.Compared with the surface topography measured by white light interferometry, the quasi-Brewster angle offset and the slope of the phase-angle change curve accurately reflect the changes of surface quality during polishing, indicating that the quasi-Brewster angle method has a high sensitivity to the surface roughness and subsurface damages of the sample. This also implies the possibility of the proposed method in characterizing the changes of the surface quality under polishing progress. Finally, the future development trend of subsurface damage detection technology for polishing process.