Abstract: In order to understand the mechanism of progressive cavity pump dropout, recreate its working status under actual production conditions underground, the pump dropout was subdivided into “internal slippage” and “external dropout” according to the pressure transmission characteristics of fluid. Slip velocity of each cavity in progressive cavity pump was calculated based on determining pump internal pressure distribution, and then the changing law of pump dropout under different suction inlet gas void ratio and outlet pressure conditions. Results show that: the change of pump inner pressure leads to fluid slippage in pump, which affects pump internal pressure distribution conversely; when “internal slippage” reaches to a certain level, it transforms into “external dropout”; There is a relative smaller “slippage region” for gaseous phase, and with the ascension of outlet pressure, the “slippage region” offsets to suction inlet; The increase of suction inlet void ratio changes the condensability fluid in pump, resulting in liquid phase slip velocity curve evolves into parabolic type from linear, and gaseous phase “slippage region” spreads to outlet. This method offers basis for determining leakage of progressive cavity pump.