To elucidate the regulation mechanism of a long-short blade runner on the internal water-sediment two-phase flow and its consequent effect on sediment erosion characteristics, a numerical simulation study was conducted on sediment motion and erosion patterns under rated generating conditions for a specific pump-turbine model. The investigation was based on the Euler-Euler two-phase flow model and the Zhang erosion model. The results indicate that erosion on the stay vanes is concentrated at the leading edge, primarily governed by high sediment concentration and direct impingement. For the guide vanes, the leading edge and the front-middle section of the pressure side constitute the critical zone for high-velocity erosion, where flow speed exerts a significant influence. The runner blades exhibit highly non-uniform erosion patterns. Erosion on the pressure side shows a strong positive correlation with the sediment concentration distribution, while the mid-to-rear section of the suction side experiences severe wear due to the secondary acceleration of particles. Furthermore, the erosion gradient is more pronounced on the surfaces of the long blades. This study clarifies the coupling relationship between erosion, local flow structures, sediment concentration, and velocity distribution across various flow-passing components and provides a theoretical basis for the anti-erosion design and safe operation of this type of unit.