Particle Impact Drilling technology (PID) is a new drilling technology, which is designed especially to solve the oil and gas exploration under hard terrane. In PID system, the steel particles were added in the drilling fluid to impact rock. The particles would be recycled and put to use again, thus it is of great significance to adjust proper drilling fluid flow rate for steel particle’s reflux. The flow rate of drilling fluids carrying particles is influenced by the fluid viscosity, the annular gap between drill pipe and wellbore, the particle volume fraction and particle size, etc. This paper mainly studied the influence of the annular gap and the flow rate, while the other factors keep constant. Both experimental method and dimension theory were employed in the research. Furthermore, empirical formula was proposed to describe the mechanism.