A buffering device includes a cylindrical casing and a piston module movably mounted in the cylindrical casing. The cylindrical casing includes a base plate and a shielding piece pivotally attached to the base plate. An air inlet and an air outlet are defined in the base plate. A diameter of the air inlet is greater than that of the air outlet. When the piston module is moving towards the base plate, the shielding piece is rotated to a closed position to shield the air inlet by air pressure above the shielding piece. When the piston module is moving away from the base plate, the shielding piece is rotated to an open position to expose the air inlet by environmental air pressure.