Abstract: Air—lock and caviation frequently occurs in the receiving and unloading tubing system of a rail tank truck of light oil during the operation in summer.Based on this phenomenon, investigation was carried out in many of the tank farms in China.From large quantity of operation information obtained, the calculated data are found to differ greatly from those got from practical operation. Reasons why airlock and caviation come ahead of time on poor working conditions are analyzed and listed as follows: vaporization characteristics of hydrocarbon; solution gas and entained air bubbles; bubble remaining, uneven temperture of the liquid inside the tank and tube leackage etc. Besides, the caculation theory is seen supplemented in the paper as well. In order to guarantee the system safety, and also with the consideration of the above mentioned factors, a new item called safety margin for vaporization is added to the calculation equation. In addition, to increase the remaining pressure inside the tube for safe unloading operation, a new method is put forward in the paper that compressed air is supposed to be injected into the tank, a submerged pump and jet pump etc can be allocated on the loading arm suction as the ancillary unloading measures so as to avoid airlock and caviation.