An exercise machine comprising a first rotating shaft, a damping device and a first changeable damping mechanism which are disposed at a front end of a main body of the exercise machine, wherein: one side of the first rotating shaft being provided with a first shifting wheel, stepped grooves of different diameters being provided on the first shifting wheel and sized down; the damping device being disposed on a damping shaft, the damping shaft being disposed close to the first rotating shaft and located at the front end of the main body, the first rotating shaft and the damping shaft being disposed on the main body of the exercise machine, the first rotating shaft and the damping shaft being parallel and spaced apart from each other; the first changeable damping mechanism including a first speed control mechanism disposed close to the first shifting wheel and a first transmission rope, the first transmission rope which having stretch elasticity and being wound on the first shifting wheel and the damping shaft; the first speed control mechanism including a first support, at least one first pull rod, at least one first spring, a first push rod, at least one first pull rope, and a rope fastener; the first support being fixed to the main body, the first pull rod being axially inserted through the first support, a front end of the first pull rod being connected with the first pull rope, a rear end of the first pull rod being sleeved with the first spring, the first push rod being fixed to the rear end of the first pull rod to hold against the first spring; the rope fastener being fixed on the first pull rod to press the first transmission rope wound on the first shifting wheel and the damping shaft, the rope fastener being interlinked with the first push rod; and a first knob connected with the first pull rope for controlling advance of the first pull rod, after released, the push rod biased by the first spring being returned, the first transmission rope being shifted on the stepped grooves with different diameters of the first shifting wheel to provide a rotational speed of different gear ratios to drive the damping device on the damping shaft for damping control. one side of the first rotating shaft being provided with a first shifting wheel, stepped grooves of different diameters being provided on the first shifting wheel and sized down; the damping device being disposed on a damping shaft, the damping shaft being disposed close to the first rotating shaft and located at the front end of the main body, the first rotating shaft and the damping shaft being disposed on the main body of the exercise machine, the first rotating shaft and the damping shaft being parallel and spaced apart from each other; the first changeable damping mechanism including a first speed control mechanism disposed close to the first shifting wheel and a first transmission rope, the first transmission rope which having stretch elasticity and being wound on the first shifting wheel and the damping shaft; the first speed control mechanism including a first support, at least one first pull rod, at least one first spring, a first push rod, at least one first pull rope, and a rope fastener; the first support being fixed to the main body, the first pull rod being axially inserted through the first support, a front end of the first pull rod being connected with the first pull rope, a rear end of the first pull rod being sleeved with the first spring, the first push rod being fixed to the rear end of the first pull rod to hold against the first spring; the rope fastener being fixed on the first pull rod to press the first transmission rope wound on the first shifting wheel and the damping shaft, the rope fastener being interlinked with the first push rod; and a first knob connected with the first pull rope for controlling advance of the first pull rod, after released, the push rod biased by the first spring being returned, the first transmission rope being shifted on the stepped grooves with different diameters of the first shifting wheel to provide a rotational speed of different gear ratios to drive the damping device on the damping shaft for damping control.