Abstract: Air-lock easily occures due to the high temperature in summer and the lower liquid level during storage and transportation of light oil by means of rail tanker. Air-lock will influence seriously normal unloading operation and increase evaporation loss. Through analyzing the causes of air-lock occured, it is pointed out that the air-lock is mainly influenced by saturated vapor pressure, flow rate and installation height of loading arm, the most effective method to eliminate air-lock is to restrict the installation height of loading arm. With perfect practicality and economy, the allowable value of the extreme installation height for loading arm is given on no air-lock condition at certain temperature and flow rate, and also given is a pro-