In order to study the effect of start-up combinations on the flow field characteristics of water-sediment in the inflow forebay of multi-sand water source pumping station, taking a typical pumping station in Jingdian irrigation area as a research object, the three-dimensional geometric model of the forebay of pumping station in the siltation condition was reconstructed by using reverse engineering technology based on the Realizable k-ε model and the Mixture model considering the phase slip. The numerical simulation of water-sediment two-phase flow in the in-situ pumping station forebay is carried out using FLUENT software and the reliability of the numerical model is verified by comparing the simulation and measured results. Based on this, the numerical simulations of the water-sand flow field characteristics of the pumping station forebay under non-siltation conditions with different start-up combinations were carried out. The results show that flow pattern disorder and serious sediment deposition exist in the forebay of in-service in-situ pumping station. The numerical simulation of flow pattern in in-situ forebay agrees well with field measurement results. The significant mainstream effect and a large low speed recirculation zone exsit in forebay of in-service prototype pumping station. With the start-up of the units at both ends of the pumping station, the diffusion effect of the mainstream in the forebay is improved, the flow field distribution tends to be symmetrical, and the sediment silt situation is slightly improved.