Abstract: Objective Explosive pits and liquid metal layers generated during rail flash butt welding tend to induce the aggregation of internal impurity particles and form oxide inclusions, thereby impairing welding quality, this study is intended to investigate the flow laws and extrusion behavior of impurity particles. Methods Based on Fluent software and the discrete phase model, a 2D fluid flow model for liquid metal extrusion under the non-flat end surface during rail flash butt welding is established. The effects of particle position, upsetting speed and end surface flatness on the movement behavior of impurities are numerically simulated and analyzed. Results The results indicate that as the end surface roughness increases, the extrusion ability of particles within the liquid metal layer decreases from 79.5% to 64.1%. As the size of the explosive pits increases, the proportion of oxides remaining on the end surface rises from 29.1% to 48.7%, making impurities more likely to accumulate within the pits to form large gray spot defects. A higher upsetting speed consistently enhances the extrusion ability of impurities in the liquid metal layer under varying numbers of explosive pits, however, it has a limited effect on improving the extrusion of impurities trapped inside the explosive pits. Conclusions End surface roughness and large-sized explosive pits severely hinder impurity extrusion and promote the formation of gray spots. Applying a higher upsetting speed can effectively reduce oxide inclusions.