In active magnetic bearing-rotor systems, unbalance compensation is used to force the rotor to spin around its geometric axis for better rotary precision. A method, which produces the control signal according to the real-time position of the rotor's unbalance mass, is proposed to actualize the unbalance compensation. Unlike the unbalance force, the unbalance mass' magnitude and position are the inherent property of the rotor and independent of the rotor speed. While the rotor speed is varying, the unbalance mass position needs not to be recalculated continuously and frequently. It will significantly reduce the computational cost. Additionally, the seeking step size in the proposed method is a constant in order to avoid the signal disturbance, so that the control can achieve the further steady-going convergence. The control performance in the condition with different unbalance phases, different seeking step sizes and different noise levels are analyzed in simulations. The experiments indicate that the proposed method achieves the good effectiveness to suppress the rotor vibration. Furthermore, it is of benefit to reducing the computational cost and suitable for application in the varying-speed operation.