The present invention is related to a cushioning device for treadmill, wherein the treadmill has a base with a side frame on each of its both sides, a deck disposed on the base to allow for the motions of running or walking by users, and a track surrounding and disposed on the deck. The deck is fastened to the base at least at its one end. At least one cushioning devices with adjustable cushion force are correspondingly disposed between the both sides of the deck and the side frames of the base. Each cushioning device has a plunger cylinder used to provide a cushion force in the reverse direction to cushion impact loads imparted on the deck while running or walking.