Abstract: A pumping station is widely used in a water distribution system for the efficient and reliable transportation. However, the sediment deposition can pose an outstanding decrease in the operating efficiency of pump station. Particularly, the water is taken from the river with the high content of sand, such as in the Yellow River Basin. More importantly, the low flow pattern and large-scale backflow area can often occur in the forward intake forebay of a large-scale pumping station. The current anti-siltation measures cannot effectively improve the intake field structure in large-scale pumping station, due to the multiple dimensions, and single rectification. In this study, a systematic investigation was made to clarify the influence of diffusion angle on the flow field structure in the forward intake forebay of a pumping station. The typical research object was taken as the forward intake forebay of large-scale pumping station in Jingtaichuan Electric Power Irrigation Project in Gansu Province of China. A field investigation was firstly performed on the sediment deposition in the forward intake forebay of pumping station. The sampling was then implemented to analyzed the variation characteristics of sediment concentration and sediment particle size in the water flow from pumping station. A three-dimensional model was also established for the forward intake forebay with the different diffusion angle. Numerical simulation was carried out using the Mixture multiphase, and Realizable k-ε model. The actual flow measurement was utilized to verify the accuracy of the numerical calculation. The characteristics of flow field structure were clarified in the forward intake forebay, together with the variation under the diffusion angle of the forebay. The mechanism of sediment deposition was revealed to propose the diffusion angle range for the optimal flow field structure in the forward intake forebay. The results show that the flow field structure was symmetrically distributed in the forward intake forebay of pumping station under the design operation conditions of the unit. The mainstream area was formed in the center of the intake forebay, whereas, the backflow area was formed on both sides. The flow velocity and the ability of current to carry sediment in the central mainstream area were much higher and stronger than those in the backflow area on both sides. The sediment deposition decreased significantly from the both sides to the center, indicating the significant mainstream effect. Furthermore, the squeezing effect of vortices was weakened in the backflow areas on both sides of the intake forebay on the mainstream area, as the diffusion angle of forebay decreased, while the area of low flow velocity on both sides also decreased significantly. At the same time, there was an increasing trend in the width of the central mainstream area, whereas, a downward trend was found in the flow velocity in the mainstream area. The ability of current to carry sediment increased to promote the performance of the flow diffusion entering the forebay. Once the diffusion angle was between 25°and 30°, the flow entering the pool was well developed to diffuse sufficiently along the pipeline, indicating the better flow pattern with the much more stable structure of flow field . The sediment deposition was effectively reduced in the forebay of the diversion pumping station on the sandy river. As such, the operating efficiency of pumping units was improved to guarantee the realistic benefit of irrigation project. The research findings can also provide the strong reference to design and update the large-scale pumping stations.