A reverse tension mechanism for a strapping machine includes an active wheel, a supporting arm installed with a passive wheel, a lever, and a spring. The supporting arm can be moved relative to the active wheel, so that a strapping band which passes between the passive wheel and the active wheel is clamped to control retracting and tightening of the strapping band. A first end of the lever is pivotably connected to a section of the supporting arm, and the spring is vertically installed between a second end of the lever and the section of the supporting arm. The first end of the lever can be activated by a cam to press the spring vertically to produce a pushing force to swing the supporting arm.