Abstract: Air-lock, cavitation and insufficient suction capability are liable to occur in an road tank car unloading operation. After analyzing the reason of its causing, giving reasonable design of suction unit, choosing proper pumps and suitable oil transporting process coefficient, the insufficiency in road tank car's unloading will be avoided. Relevant figure is given through calculation; it provides a theoretical basis for not running in air-lock and cavitation conditions, and working in the condition of sufficient suction capability, and offers a convenience for the design of suction unit and selection of pump.