Abstract: The periodic water level regulation between 145~175 m above sea level in Three Gorges Reservoir area promotes the deterioration damage and stability attenuation of the limestone bank slope in the reservoir area. This paper is based on the example of "sliding-bending" failure of bedding limestone bank slope in Wuxia section and the model of elastic-plastic plate warping. It introduces the concept of rock mass deterioration and deduces the equilibrium equation of critical flexural section. Compared with the traditional static calculation method,the proposed method has wider applicability and can consider the dynamic evolution process of bank slope with deterioration time t. The proposed method is more in line with the engineering practice. The calculation results show that the critical deflection section l_cr gradually decreases with deterioration,and the stability of bank slope becomes worse. The sliding thrust F gradually increases and bending stiffness K gradually decreases. These two factors lead to the rapid decrease of the critical flexural segment l_cr. Since the rock mass deterioration GSI(t) is an exponential deterioration form,l_cr presents that decreases rapidly and gradually slows down to a certain value,which can be defined as the"key section" of the bank slope treatment. With the increase of dip angle α,l_cr and relative plate length ratio RPL decrease rapidly. With the increase of h_i,l_cr and RPL gradually increase. The influence of rock dip α on l_cr and RPL is obviously greater than the rock thickness h_i. Moreover,the rapid descent angle α of bank slope l_cr is about 40°~60°,which can be used as a geological criterion for similarly bedding bank slopes that are prone to sliding-bending instability. The critical balance equation deduced in this paper considering the deterioration of the rock mass is calculated and discussed through the Qingshi 6^# bank slope of the Three Gorges Reservoir. It can provide references for the damage evolution calculation of other similar bedding slopes. The difference is to determine the GSI(t)function of the specific slope damage deterioration.