A magnetron mechanism of an unpowered treadmill contains: a driving body, a one-way transmission element, and a magnetron mechanism. The driving body includes a frame, a connection fence, a front wheel assembly, and a connection shaft. The one-way transmission element is mounted on the connection fence of the frame. The magnetron mechanism is fixed on the connection fence and includes a rotary shaft, a drive wheel, a driven wheel, a belt, a flywheel, a resistance element, and an adjustment unit. The resistance element has a pair of fixing sheets and multiple magnetic parts. Each of the multiple magnetic parts has a rotatable connection portion. The adjustment unit has a steel cable, an end of which is connected with the resistance element so that the steel cable pulls the resistance element to swing along the rotatable connection portion.