The slurry pump, which forms the core equipment of the deep-sea mining (DSM) system, provides lifting power for the ore from the seabed to the sea level, which is crucial for the safety of coarse ore particle transportation. VELOCITY SLIP plays a significant role in revealing the migration of the pump particles. Therefore, this study analyzes the VELOCITY SLIP in a slurry pump using the computational fluid dynamics–discrete element method (CFD-DEM) for the first time. The relationship between the pump head and VELOCITY SLIP was proposed and verified in this study based on the VELOCITY triangle and Euler equation of the solid-liquid two-phase flow in the impeller. The effects of different particle sizes on the VELOCITY SLIP are compared in detail. According to the computational results, the head depends on the larger VELOCITY SLIP of the impeller outlet and lower VELOCITY SLIP at the inlet. The peak value of the VELOCITY SLIP was significantly reduced, and the peak position of the VELOCITY SLIP and zero-point position moved backward for particle sizes ranging between 5-15 mm. This study provides a reference for the problems of particle migration and VELOCITY SLIP in slurry pumps.