An integrated folding mechanism of a treadmill having a slide pusher supported by a pneumatic cylinder at the bottom of a platform frame thereof. In this way, the rear support may be pushed upward to bring the platform frame in a vertical stable folding position or in a flat ground-touching position when the platform frame is moved upward and downward. Meanwhile, a coupling rod is pivotally interposed between the rear support and an L-shaped handrail assembly. Under the action of an elastic element, the handrail assembly is movable with the platform frame for creating a synchronic folding and extending action.