A support device includes a base, a support arm, an air spring unit, and a hinge. The base is mounted on a reference surface; a first end of the support arm is pivotally connected to the base; the air spring unit is disposed in the support arm, and includes a switch assembly having a rotatable cam and an air spring, the cam has a first and a second lock points; the air spring provides a supporting force to the support arm, a tube thereof is disposed in the support arm, a piston rod thereof is telescopically disposed at a first end of the tube, and a release pin thereof is disposed at a second end of the tube. By adjusting the release pin abuts against the first lock point, a position of the hinge can be adjusted and then stopped at any position between the highest and lowest positions.