Subsurface damage (SSD) may still exist in polished elements, which influences laser induced damage threshold and optical performance. Therefore, SSD must be reduced and finally removed from the polished surface to improve the service performance of optical elements. Hence it is required to make the accurate detection and characterization of the polishing SSD. In the current approach, firstly, the depth of hydrolyzed layer and embedded polishing contaminants were detected with constancy chemical etch rate technique and secondary ion mass spectrography, respectively. Then, dimension of subsurface plastic scratches was measured with atomic force microscope. Furthermore, the development of surface roughness with depth was analyzed, in order to discuss the correlation among superficial flow layer, hydrolyzed layer, and subsurface plastic scratches. Finally, the SSD model in polishing process was established, and, material removal mechanism was proposed on the basis of the model. The results show that hydrolyzed layer contains superficial flow layer, plastic scratches, and polishing contaminants. The depth of hydrolyzed layer on polished fused silica surface ranges from 76nm to 105nm. Optical polishing process is responsible for the corporate effects of hydrolyzed action, mechanical removal, and plastic flow.