The invention relates to a coaxially-driven assistant torque generator of an under-actuated system, belonging to an electromagnetic torque-type torque generator. The coaxially-driven assistant torque generator is characterized in that the assistant torque generator comprises a case (1), a magnet group (2), an armature winding (3), a front end cover (6), a motor (8), a motor seat (10) and a load (11), wherein the case is coaxially fixedly connected with the under-actuated system. Based on the electromagnetic induction principle, the motor drives the armature winding to rotate in the magnetic field to generate an electromagnetic torque in a direction opposite to the motor driving direction; and the magnet group is applied with an inverse electromagnetic torque having the same intensity and opposite direction in relative to the armature winding, and transmits the inverse electromagnetic torque to the under-actuated system to provide desired torque. Particularly, the torque provided by the generator is in direct ratio with the velocity item; the control difficulty is lowered remarkably, and the assistant torque generator can realize two-stage electromagnetic speed regulation. The coaxially-driven assistant torque generator provided by the invention is simple and reliable in system design and clear in structure, can be applied for adjusting attitude of one-wheeled robot, satellite, space shuttle and missiles and also can be applied for overcoming reactive torque of helicopters or saucer-type flight vehicles, and so on.