In an aero-engine, which has small rotor-casing clearance and delay in speed control, coupled bending and torsional vibration may occur due to rotor-casing contact. A rotor-casing contact model of a flexible rotor with elastic bearing and elastic stator was established, and the gyroscopic moment of the rotor and the delayed rotational speed control torque was considered. Then, the rotor′s coupled bending and torsional vibration were studied using a numerical method. The results showed that when the rotor-casing contact event occurred, the backward whirl was caused by violent rotor-stator contact. Under the effect of the rotor-casing contact and the delayed rotational speed control torque, the torsional vibration occurred and caused the instability of the torsional vibration. The larger the rub torque, the more easily the instability of the torsional vibration occurred. This rotor dynamic effect should be considered in the control system design of an aero-engine, and the torsional vibration signal of the rotor can be used as important diagnostic information of the rotor-casing contact.