Abstract: The shaft-inflow pumping stations are frequently design-constrained due to the urban and rural planning, and land use limitation. The phenomena such as mainstream deviation and backflow are prone to occur, as a result of poor inflow conditions in the design-constrained shaft-inflow pumping station, harmful to the safe, stable and efficient operation. The shaft-inflow pumping station distributes water to the forebay through the bottom pipe culvert and the inlet shaft, resulting in complex flow conditions in the forebay. The water flow in the forebay and inlet sump often has the characteristics of concentrated mainstream, uneven velocity distribution in the vertical direction. To improve the flow pattern of the shaft-inflow pumping stations, this study adopts a method based on numerical simulation and supplementary verification by physical model tests to analyze the causes of the poor flow pattern in the forebay of a shaft-inflow pumping station under the rated operating flow rate of 28.92 m³/s. The inlet boundary is set as the pressure inlet, the outlet boundary is set as the mass flow outlet, and the walls are set as the non-slip wall. For water surface, by comparing the numerical simulation results of the volume of fluid (VOF) method and the rigid-lid hypothesis method, and considering the calculation cost and research efficiency, the rigid lid hypothesis method is ultimately adopted to simulate the water surface. Qualitative and quantitative analyses are conducted using rotation correction factor, velocity uniformity, and global reflux coefficient. The rectification effect of various types of diversion pier on the flow pattern in the forebay are compared. Using the response surface methodology (RSM), the final optimization scheme of diversion pier is recommended. The research results show that due to the influence of the bottom inflow and excessive diffusion angle, the significant mainstream diversion appears in the forebay and a large-scale reflux zone is formed, resulting in poor hydraulic performance of the pumping station. Compared with vorticity, the rotation correction factor can be used to effectively characterize the large-scale reflux zone in the forebay of the pumping station. By comparing the rectification effects of diversion piers setting in the front of the forebay and inside the box culvert, the diversion piers mainly adjust the flow pattern in the forebay by guiding the mainstream to uniformly spread in the forebay. Arranging diversion piers inside the box culvert can more effectively improve the flow pattern in the forebay. Based on RSM, the optimization results of diversion pier setting in the box culvert reveals that the interaction between the distance between the head of diversion pier and the outlet of box culvert X and the angle between the diversion pier and the pumping station θ is relatively strong. The extremum of the objective function S is solved within the ranges of 1.61975D ≤ X ≤ 1.83501D, 0.660369D ≤ b ≤ 0.665309D, and 7.47556β ≤ θ ≤ 8.25672β. The final optimization combination of the diversion pier is: the distance between the head of diversion pier and the outlet of box culvert X is 1.74D, the distance between the head of diversion pier and the inner wall of box culvert b is 0.66D, and the angle between the diversion pier and the pumping station θ is 0.333β. The velocity uniformity and the global reflux coefficient of the final optimization scheme are increased by 28.02% and 13.72%, respectively, compared with the design scheme without diversion pier. Finally, the numerical simulation results are verified through physical model tests. The surface flow pattern in numerical simulation is similar with the that in physical model test. The relative errors of hydraulic loss in the forebay and mean velocity are less than 5% and 9%, respectively, indicating that the numerical simulation method is accurate and the results of the optimization scheme are reliable. The research results can provide theoretical references and technical support for both the design and safe and stable operation of the shaft-inflow pumping station.