Abstract: Loading and unloading from the top of a light oil rail tank has been the main method in China so far. There remained many problems difficult to solve such as air lock, vapor loss higher than the standarized, complex operating procedures, too much labour involved and being less safe. etc. To solve the problems, a bottom unloading device is designed which is of air-seperation type with grease as its sealing material. The device is made up of a sealing piston, an actuator, and a piston cylinder. Both the sealing piston and its cylinder form the grease chamber and the air chamber. The latter is filled with grease, both ends are sealed with sealing rings. The air chamber seperates the product oil from grease. Compared with a valve, this device has the following five characteristics: 1. Generally, the sealing rings, medium being sealed, the width of the rings and the reaction force acted on them can be regulated. 2. The sealing material is composed of grease and polymer, not only stiff but also good for filling. 3. The medium is sealed with the sealing piston on which there are both a grease chamber and an air chamber. The product can be fully sealed by means of absolute sealing of the air in the air chamber. 4. No maintenance is necessary during application except regulating the pressure and grease level in the grease chamber. 5. This device is suitable only for low temperature environment.