Abstract: A 65ZW30-40 sewage self-priming pump is numerically modeled to study its flows of solid particles and their distribution patterns focusing on the effect of different particle diameters. The solid-liquid two-phase flows in the pump are simulated using ANSYS Fluent together with a multi-phase flow model the Mixture and a RNG k-ε turbulent flow model. Results show that as the particle diameter increases, the lift head and efficiency of the pump decreases gradually and the decrease in pump performance is significant. Most solid particles are distributed near the trailing edge of the blade suction sides, the outer pump wall, and the back-flow hole inlet. Simulations also reveal that a larger back-flow hole can make the particle pattern closer to uniformly distributed, while a smaller hole results in more particles concentrated on its both sides, near the trailing edge, and over the intersection of impeller and volute. The size of back-flow hole has a crucial effect on its flow rate and thus the particle distribution pattern is sensitive to its variation. As it increases, the particles concentrated zone migrates toward the volute, and the particle volume fraction in the core of the back-flow hole decreases gradually.