The invention aims at providing a three-level piston buffering type hydraulic drive vent valve mechanism. The three-level piston buffering type hydraulic drive vent valve mechanism comprises a vent valve, a drive unit and a hydraulic oil pipe. The vent valve and the drive unit are communicated through the hydraulic oil pipe. The drive unit comprises a drive piston set. The drive piston set comprises a first-level drive piston, a second-level drive piston and a third-level drive piston. The third-level drive piston is embedded in the second-level drive piston. The second-level drive piston is embedded in the first-level drive piston. When the drive piston set ascends, the first-level drive piston reaches the lift limitation, the first-level drive piston stops moving, and the second-level drive piston and the third-level drive piston are kept ascending; and then the second-level drive piston reaches the lift limitation, the second-level drive piston and the third-level drive piston are separated, and the third-level drive piston is kept ascending till reaching the extreme position. The exhaust timing controllability is achieved, and the lift fluctuation of the maintaining stage of the vent valve can be stabilized on the premise that the high starting and stopping rate of the vent valve is ensured; and the moving impact force of all components of the vent valve is reduced, so that the service life is prolonged.