A torque sensor has an envelope tube, a middle cylinder mounted in the envelope tube, a inner cylinder mounted through the middle cylinder, a resilient element connected to the middle cylinder and the inner cylinder, a first disc and a second disc respectively attached to the inner cylinder and the middle cylinder, and a detection unit detecting a displacement angle of the second disc and the first disc to calculate a torque applied to the inner cylinder. The torque sensor has a simple structure, fast, sensitive and accurate torque sensing ability, and can be used on all kinds of mechanisms that rotate when a force is applied thereon to calculate and quantify amounts of energy and work of the exerciser according to the torque.