Three-dimensional hydro-mechanical coupling analyses have been conducted on the water-tight structure of the cofferdam for both intake and outlet of Taian pumped storage power station, located in Shandong Province of China. In addition, the effects of excavation on the cofferdam and foundation slopes were also studied by using the 3D FLAC. The calculation results show that the central core of high-pressure grouting has a prefect anti-seepage effect and therefore is able to strengthen the stability of the cofferdam and foundation slopes. The excavation process has only some local effects on the cofferdam and does not greatly affect the global stability of the cofferdam. Therefore, no failure takes place around slope toes. The results show that the width of platform left on the excavation side is reasonable.