Traveling wave ultrasonic motor (TWUSM) is driven by friction force between stator and rotor, so friction coefficient is an important parameter of contact interface. In fact, there have mutual influence and restriction in various parameters of contact interface. For a comprehensive study on friction coefficient, the interaction between friction coefficient and other parameters should be considered. Base on a new contact model with visco-elastic stator contact layer of TWUSM，coupling effects of two parameters on motor output characteristics were simulated with MATLAB. According to the simulation results, a design principle of friction coefficient was present. A kind of Ekonol composite friction material (EK2) was developed under the guidance of this principle. Comparing with other common friction material of TWUSM, it was proved that motor with EK2 can get better running stability, higher output torque and longer lifetime. Therefore, EK2 is a good friction material which can satisfy the actual running needs of TWUSM.