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Sediment erosion of centrifugal pump impeller is a major issue of pumping stations on the Yellow River. The sediment erosion at the outlet of the impeller blade suction side of a double-suction centrifugal pump was mainly analyzed by experiments and numerical simulation. In the experiment, the blade sediment erosion was visualized by using the multi-layer coating method and the near-wall flow pattern were observed by the tuft visualization method coupled with the endoscopic imaging technique respectively. The experimental results showed that there were two triangular severe erosion regions symmetrically distributed at the outlet of the suction side of the blade. In the same regions, the flow separation can also be observed clearly. Furthermore, the Euler-Lagrangian method was used to analyze the mechanism for the sediment erosion on the two triangular severe erosion regions. The inter-blade vortex in the pump impeller and the recirculation vortex at the blade outlet were considered responsible for the formation of severe erosion. The inlet-blade vortex can guide sediment particles to aggregate and hit the outlet of the blade suction side. In addition, the recirculation vortex near the outlet of the suction side of the blade made the particles hit the blade repeatedly. Their combined effects lead to severe sediment erosion on the outlet of the blade suction side. The research results can supply theoretical reference for anti-erosion design of centrifugal pump.