In Mengdigou Hydropower Station, faults and joint fissures are developed in the plug section of diversion tunnel. The geological conditions are poor, the water head of the plug is high, the boundary conditions are complex, and the plugging period is short. Therefore, it is urgent to study the stability of the whole process of diversion tunnel excavation, construction and operation. The three-dimensional nonlinear finite element method was used to simulate the whole process of diversion tunnel excavation, lining construction, plug construction and plug operation. The following conclusions are drawn: (1) During the excavation process of the tunnel, the surrounding rock is basically stable, and local instability may exist in the parts with poor surrounding rock. It is suggested to take measures such as anchoring and grouting locally; (2) During the operation period, the surrounding rock and plug are overall stable, the lining structure meets the requirements of compressive strength, and the tensile stress of partial lining exceeds the tensile strength, so reinforcement should be strengthened; (3)The thrust sharing ratio of upstream section of plug with length of 40m and 50m is basically the same, and the length of the plug is recommended to be more than 40m. The research results can provide reference for the design and construction of diversion tunnel plug of Mengdigou Hydropower Station.