A cushioning device for umbrella includes a first gear, a plate spring, a bolt, and a second gear collectively arranged in a hollow chamber defined between a side cover and a casing. The umbrella has a central post to which an axially-extending toothed rack is mounted. The side cover and the casing of the cushioning device are mounted to the runner of the umbrella to have the second gear mating the rack of the central post. The first gear and the second gear of the cushioning device mate each other. The first gear has a surface to which the plate spring and the bolt are mounted. Rotating the bolt to different extents of tightness makes the plate spring apply different depression forces to the first gear, which changes the frictional resistance between the first gear and the casing. The frictional resistance is transmitted through the second gear and the rack to the central post to form a cushioning resistance between the central post and the runner, making the opening operation of the umbrella gentle and safe.