Based on finite element software Plaxis, this paper researches the stability of diseased retaining wall of reservoir bank that reinforced by pile-plank structure, and analyses the change rules of its safety factor with water level. The results show that using pile-plank structure to reinforce the diseased retaining wall of reservoir bank can effectively control the displacement of wall, obviously improve the safety factor. And the safety factor does not always reduce with the rise of water level , but presents a significant sunken type, that is the most adverse water level between the low water level and high water level, it can be find out with finite element method. The method proposed in this paper overcomes the shortcomings that safety factor of the retaining wall is not minimum for choosing wrong water level, and it is proposed to be used in calculations of important hydraulic structures.