The invention discloses a pneumatic compacting self locking type oil tank gas recycling and sealing device arranged on the top of a train. The pneumatic compacting self locking type oil tank gas recycling sealing device is characterized in that a rubber gasket is placed above a tank port, a bottom withholding mechanism is positioned in the tank port, and a vertical pipe lifting air cylinder is arranged in the bottom withholding mechanism; the vertical pipe lifting air cylinder can drive a vertical pipe to connect with the withholding mechanism, and spring bolts and a bottom support arranged in the withholding mechanism are connected with a guiding sliding groove in an outer cylinder through a sliding mechanism; when the vertical pipe drives the withholding mechanism to upward move, the spring bolts can horizontally unfold and can upward compact a tank wall, and the stable reliable pressing force is provided for the rubber gasket arranged on the upper part of the sealing device. The sealing device disclosed by the invention has the advantages that the compacting force is only acted between a sealing cover and the tank port, the influence from the rigidity of a crane pipe and a tank car is small, the provided compacting force greatly exceeds the pressured generated by oil gas, and the sealing effect is very reliable; the sealing device finishes compacting and loosening through a pneumatic assembly; the sealing device is suitable for sealing tank ports of multiple train tanks, the operation is simple, and the sealing device is used for sealing and recycling the top truck loading of a volatile oil product or chemical medium train tank of an oil refinery, a storage field or a chemical engineering unit.