Abstract: The paper analyzes the causes of air-lock and pump off of centrifugal pump produced during the light ends unloading operation of rail tanker. To solve this problem.the measures to eliminate the air-lock and prevent centrifugal pump from evacuating are presented, that is, controlling the vacuity of gas gathering tank with the technology of vacuum-serving unloading.and adjusting the two absolute pressures at the climax of unloading arm and the inlet of centrifugal pump. An economic comparison is made between the vacuum-serving unloading technology and positive pressure unloading. The results show that the vacuum-serving unloading technology has obvious advantages in high safety, low power consumption and convenient operation.