The work aims to propose a detection method of the mechanically processed subsurface damage depth of optical glass to provide reference for the ultra-precision polishing depth of optical glass. Firstly, the microstructural evolution of the subsurface crack of K9 glass during HF/NH4F etching was studied, the crack depth of etching surface was detected by probe roughness tester, and the influence of probe radius and etching time on subsurface crack depth was discussed. Therefore, the measuring requirements for characterizing subsurface crack depth based on surface peak-to-valley (PV) roughness curve were established. Then, the PV roughness of etching surface was measured by laser scan confocal microscopy to determine the subsurface crack depth of optical glass. Finally, the subsurface damage depth was measured directly by cross-sectional polishing method to verify the reliability of the above two detection methods. The measurement condition of characterizing the depth of subsurface cracks by PV roughness of etching surface was that the measuring medium must effectively detect the bottom of cracks before the cracks on etching surface began to fuse. For the K9 glass samples lapped with W18 and W40 abrasives, subsurface crack depth of these two samples detected by the method based on etching process and laser scan confocal microscopy was 12.82 μm and 20.45 μm, while the results detected by the cross-sectional method were 12.50 μm and 19.34 μm. The deviation between these two methods was 2.56% and 5.74%, respectively. The obtained results were in concordance in the both used methods. Furthermore, the methods based on chemical etching process and laser scan confocal microscopy are not limited by the width of surface crack, and also meet the requirements for characterizing subsurface crack depth with PV roughness curve, with less damage on samples, which improves measurement efficiency and result reliability of the subsurface damage depth of optical glass.