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 connecting rod is interposed between the rear support and the handrail assembly such that the handrail assembly can synchronically move with the platform frame in an extracted or an extended position.