The invention discloses a reverse variable displacement fluid mechanism. The reverse variable displacement fluid mechanism is characterized in that a rotor shell is arranged in a machine shell in a swinging manner, a rotor with sliding pieces is arranged in a rotor shell, a cover body A and a cover body B which are in sealing fit with the rotor shell and the rotor are arranged on the two sides ofthe rotor shell and the machine shell in the axial direction, the sliding pieces, the cover body A, the cover body B, the rotor and the machine shell are in sealing fit to form a volume change area, the machine shell is correspondingly assembled with the cover body A and the cover body B in a sealing manner, a sliding sealing area is arranged between the rotor shell and the machine shell, the sliding sealing area, the rotor shell as well as the swinging interface of thee machine shell, the cover body A and the cover body B form a sealing cavity, a reset elastic body is arranged in the sealingcavity in a thrust direction to push the rotor shell to deflect to enable the eccentric distance between the rotor shell and the rotor to be reduced, and the sealing cavity communicates with a workingmedium opening of the fluid mechanism formed by the rotor, the sliding pieces and the rotor shell. According to the reverse variable displacement fluid mechanism, the external cavitation of an engineair cylinder can be greatly reduced, and the pumping consumption of an engine cooling system can be reduced or eliminated.