Based on Three Gorges underground power station, a numerical prediction model with pressure & free-surface mixed flow and a complete simulation model test stand with model hydro-turbine generator unit for hydraulic transient process have been set up. By means of numerical simulation calculation, complete simulation model test with model hydro-turbine generator unit, prototype hydrogenerator unit field monitoring and inverse calculation, a new structure pattern with tailrace tunnel of slopping ceiling and damping tank for Three Gorges underground station giant hydro-turbine generator unit has been proposed and verified, which resolves effectively the complex technology problem that the giant surge chamber cannot be built as the poor geological conditions. Therefore, the successful construction of hydropower station and stable operation of hydro-turbine generator units have been guaranteed. The research results provide reliable method and basis for improving and optimizing the numerical simulation model for unsteady flow in the tailrace tunnel of slopping ceiling and Three Gorges giant hydro-turbine generator unit commissioning and steady operation control strategy. Moreover, it can provide reference for hydraulic transient process research of other hydropower station with complex water conveyance system.