A power-operated steering assisting device is provided for a vehicle and includes a rotation detector, a motor assembly, and a reduction and driving mechanism coupled to a steering shaft of a steering wheel of the vehicle. Upon detecting the rotation of the steering shaft, the rotation detector activates the motor assembly to drive the operation of the reduction and driving mechanism, so that through the engagement between an internal-toothed output ring of the reduction and driving mechanism and a toothed section formed on an outer circumference of the steering shaft, the steering shaft is rotated and an assistance to the steering operation carried out by a driver of the vehicle is realized to provide an effort-saved operation of turning the steering wheel by the driver. No hydraulic system is included in the steering assisting device and thus maintenance of the hydraulic system can be eliminated.