A side-supporting type folding mechanism for a treadmill having a rotating shaft pivotally disposed at the bottom of a base frame of the treadmill. Both sides of the rotating shaft are provided with a wing panel. The other end of the wing panel is supported by a pneumatic cylinder. Meanwhile, the center of the wing panel is pivotally connected with a top rod such that another end of the top rod pushes a rotating disc movable with the handrail. In addition, the center of the rotating shaft is connected with an auxiliary support having ground-touching rollers. Meanwhile, the pivot joints among the pneumatic cylinder, the wing panel and the top rod are disposed at a position of the same axle line. In this way, a potential great shear stress created between the supporting elements can be eliminated when the treadmill is folded in a folded position. Thus, the service life of the supporting elements can be extended.