Taking the flexibility of rotor into account, a three-dimensional surface contact model of ring type traveling wave ultrasonic motors is developed. The stator vibration, the normal contact and the tangential friction between the stator and rotor are analyzed, and the surface contact mechanism between the stator and rotor and the torque-speed characteristics of the motor are investigated. The results show that owing to the flexibility of the rotor, the shape of contact area between the stator and the rotor is irregular and the contact pressure distribution along the radial direction is inhomogeneous. The three-dimensional surface contact model is validated by comparing the calculated torque-speed characteristics with the measured values of the prototype motor. Based on the three-dimensional surface contact model, the influences of the structural parameters of the flexible rotor on the maximum contact pressure and the torque-speed characteristics of the motor are analyzed. The structure of the rotor is optimized according to the analysis results, which achieves a significant reduction in the maximum contact pressure as well as an obvious improvement of torque-speed characteristics of the motor.