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, the rear support is provided with a push rod opposing to the handrail assembly. The push rod is adapted to push a coupling cross shaft having a rotation disc at both sides thereof, respectively. In this way, the handrail assembly fixed coaxially to the rotation discs can be synchronically moved in a flat position or in an upright position.