Abstract: Based on the deep trench excavation works of the dam foundation of the Longkaikou hydropower station, the reinforced concrete underground wall for retaining water with four-node thin-slab element is simulated by a finite element analysis method in this study. A finite element analysis model for supporting stiffness coefficients of the foundation soil mass on the upper side of the underground wall for retaining water and its supporting structures is developed in the study. Based on the analysis, a determination method of the supporting stiffness coefficients of the underground wall for retaining water is put forward, and the analysis of the reliability of the impervious wall under different support conditions is carried out. The analysis and simulation results show that the changes of the impervious wall displacement are larger than the internal force, and that the support conditions have great influences on deformation of the impervious wall under the different support conditions. The research results given by this paper can provide a reliable theoretical basis for the construction and monitoring of the deep groove impervious wall located at the dam foundation of the long open hydropower station, and a reference for the similar hydropower works.