The evolution of the rocky reservoir bank is complex and difficult to predict because there is a lack of connection between changing mechanical states and reservoir water levels. Here, we propose an analytical method to study the evolution process of thick-layer dangerous rock masses (TDRM) on reservoir banks. This new method involves damage evolution and numerical simulation. Through this new method, the non-linear accelerated mechanical state of the TDRM can be continuously tracked, and the relationship between the changing water level and the progressive failure can be quantified. A dynamic collapse analysis can be carried out using parameters obtained from a predictive model. In this paper, the Jianchuandong dangerous rock mass (JDRM) located in the Three Gorges Reservoir area is selected as a case study to verify the new method. The predictive model was highly consistent with long-term monitoring and observational data. In addition, the failure process obtained by numerical simulation was identical to the typical dynamic characteristics of TDRM. Therefore, the effectiveness of this new method has been proven, and the method can be used to predict the evolution of TDRM on other reservoir banks.