Abstract: The long-distance,non-pressure branch tunnel is a hydraulic structure that affects the flow regime and discharge ratio. In the water-filling stage,there are complex flow regimes in the branch tunnel. In this paper,the numerical simulation calculation in the non-pressure branch tunnel at the water filling stage,considering different inlet flows and diversion angles,was carried out by the FLUENT software. At the same time,the water depth at each monitoring station and the discharge ratio for the branch tunnel,after the water flow is stable for different inlet flows,were verified by the model test. The results show that the inlet flow during the water filling stage has little effect on the discharge ratio,which depends mostly on the diversion angles,between 15° and 45°,considered in the simulation. In addition,a recirculation zone appears in the upper flow outside of the lateral branch when the diversion angle is increased to 60°. The change in the diversion angle has little effect on the main tunnel water surface line. The water level in each tunnel section increases when the inlet flow increases.