In the rotor-bearing system fault diagnosis, the data mainly come from bearing vibration monitoring. When investigators studied the coupled flexural and torsional vibration, they only took into account the bending vibration, the torsional vibration and swinging vibration. But it’s plagiohedral for them to only monitor bearing vibration signals. This paper presents a circumferential loading system which the torque is imposed on the rotor - bearing system by couplings. On the system we studied the disturbance of rotor-bearing system caused by circumferential loading torque, and observed that the bearing vibration response was inconsistent with the bearing load changes. This provided a new basis for the rotor-bearing system fault diagnosis.