This paper puts forward an analysis method for the dynamics of the contact interface of travelling-wave rotary ultrasonic motors (TRUMs) by discreting the stator/rotor interface to avoid the complexity non-linear deduction of the motor in the recent researches. First, the surface-contact of each stator tooth was equivalent to point-contact, for which the concept of equivalent stiffness of different contact states was proposed to analyze the contact states. The discrete axial forces between the stator teeth and rotor were described as the equivalent stiffness, and the equivalent force between the stator and the rotor was obtained. Thus, the relative movement of the stator and the rotor was calculated, and the axial dynamics characteristics of the stator and the rotor were gained. Then, the radial and circumferential friction was given by the three-dimensional motion in the interface to study the mechanical characteristics and the energy consumption. The final simulations and experiments demonstrated the accurate description of the interface dynamics by the proposed method.