Considering that the control accuracy of travelling-wave rotating ultrasonic motors(TWUSMs) is directly related to the two-phase vibration modes of the stator, but which are not easy to measure, a sliding mode observer(SMO) with parameter robustness is proposed to optimize the stator vibration modes. Firstly, the influences of the two-phase vibration modes of the stator and the travelling wave to the control accuracy of TWUSMs are analyzed.Secondly, the SMO with vibration modes is established using the measurable current and voltage signals, and then its stability and robustness are proved under the time-varying parameters. Finally, the proposed sliding mode observer with vibration mode is verified through simulation and experiment. The results show that the SMO can accurately estimate the two-phase vibration modes and has the parameters robustness. In addition, based on the observation results, the driving voltage is improved, and the stator vibration mode is optimized, thereby reducing output torque ripple and improving motor control accuracy.