A steer-by-wire system uses a semi-active actuator in which a wheel steering motor operates vehicle wheels when a steering wheel is rotated. The steering system includes: a steering operator unit having a steering column with one end vertically connected to the center of the steering wheel, a torsion spring connected between the other end of the steering column and a vehicle body via jigs mounted to both ends thereof, a magneto-rheological brake for providing steering reaction force according to the amount of current, and rotation angle sensor coupled with one end of the steering column for detecting rotation of the steering wheel, and a torque sensor coupled with one end of the steering column for detecting torque of the steering wheel; a current regulator for controlling resistance force against rotation of the magneto-rheological brake.581 A controller is provided for controlling the current regulator according to torque data from the torque sensor, and controlling the wheel steering motor according to rotational displacement data from the rotation angle sensor.