The deterioration and loosening of rock mass in the water level fluctuation zone of the Three Gorges Reservoir area accelerates the instability of the bank slope causing serious concern for its potential to trigger an engineering catastrophe effect on the karst rock bank slope. In this study, an analogy method is proposed to quantitatively analyze the deterioration of rock mass by using the rock mass near the elevation of 175 m to replace the rock mass unaffected by impounding. The rock mass deterioration variable Det, the increment of rock mass volume joint number ΔJv and the Variation of geological strength index ΔGSI is defined, improved and counted. The deterioration index of 10 typical karst rock masses in the Three Gorges Reservoir area for 12 years since the experimental impoundment of 175 m above sea level in 2008, and its influence on the geological strength index are quantitatively analyzed. Different deterioration degrees can be corresponding to different quantitative index ranges. For example, when Det≥0. 05, ΔJOv ≥0. 65 or ΔJv≥1, the deterioration degree of rock mass is categorized as serious. The deterioration of rock mass results in the decrease of GSI; when the rock mass is seriously deteriorated, ΔGSI≤-4. 0; -ΔGSI and Det meet the exponential function relationship, ΔGSI and ΔJv satisfies the linear negative correlation. The evolution of rock mass deterioration with time may have three development modes, two of which will eventually enter the state of accelerated deterioration, and the rock mass structure will evolve into cataclastic structure through two paths. Meanwhile, the relationship between the deterioration of rock mass and the meta- instability stage is discussed. This research provides a new method for the systematic exploration, monitoring and risk control of the deterioration degree of rock mass in the water level fluctuation zone of the Three Gorges Reservoir area.