Abstract: In order to study the flow field changes of the return water exhaust system of pumped storage power station, based on SIMPLEC algorithm and Realizable k - \varepsilon turbulence model, the three-dimensional numerical simulation of the return water exhaust system of the power station was carried out, and the velocity and pressure field in the exhaust process of exhaust system under different pipe diameter were analyzed. The results show that when the diameter of the main pipe changes, the gas flow field in the branch pipe changes little, and the pressure and velocity of the gas on the main pipe gradually decrease with the increase of the diameter; when the diameter of the main pipe increases from 250mm to 400mm, the pipe flow gradually increases, and the pipe flow gradually increases from 16.79kg/s to 20.72kg/s, while the pipe flow speed decreases sharply, and the pipe flow speed decreases from 215.99m/s to 112.37m/s; at the intersection of pipes, when the diameter of the main pipe is small, the gas entering the main pipe is restrained greatly by the main pipe wall, and the gas impact on the pipe wall is small. As the diameter of the main pipe increases, the gas impact on the main pipe increases rapidly and the gas flow becomes more complicated. With the increase of pipe diameter, the gas velocity on the branch section plane1 first increases and then decreases with the increase of pipe diameter. The maximum flow rate is 284.2m/s at 300 mm pipe diameter. The gas velocity on the cross section plane2 between branch pipe and main pipe changes slightly, and the maximum flow rate is 310.8m/s at 350 mm. The gas velocity on the main pipe section plane3 gradually increases with the maximum flow rate of 362.3m/s, which occurs at the service condition of 400mm diameter.