Damping well combined with tailrace tunnel with sloping ceiling is a new type of hydropower station’s tail pipeline. There is no such operation experience for large generation units with unit capacity of 700MW in China. To learn more about the hydraulic characteristics of tailrace system with sloping ceiling of large unit, and to validate the project design and the hydraulic model test results and to ensure the safe and stable operation of the project, we conducted field monitoring on the hydraulic transition process of load rejection of TGP’s underground power station in conditions of two characteristic reservoir water levels (trial run152.3m, normal storage 175.0m) and four load magnitudes (25%, 50%, 75%, 100%). The monitoring is focused on water hammer pressure of spiral case’s end parts and tail pipe’s inlet parts, damping well surge, unsteady free-surface pressurized flow of the tail pipe, and the increment of unit rotating speed. Analysis on the monitoring results suggests that the hydraulic indicators of the tail pipe meet the design requirement, with a certain safety margin. The monitoring results could be reliable basis for the design and project operation of tail pipe in the future.