To remove the effect of environmental conditions on the measuring accuracy of HF acid differential corrosion rate method, a HF step-by-step etching method was proposed. The correction factor Ki was introduced to correct the environmental changes in different etching steps and to improve the measuring accuracy. Experiments show that the sub-surface damage layer depth of the K9 glass after Fixed Abrasive Pad(FAP) lapping is 3.479 μm measured by the HF step-by-step etching method. As compared with 0.837 μm and 2.82 μm measured by the HF acid differential corrosion rate method and the magneto-rheological finishing spot method, respectively, it demonstrates that the proposed method in this paper has higher measurement accuracy and is in well accordance with the actual situation. Furthermore, the introduction of experimental correction factor Ki reduces the accumulation errors brought by the operating environments.