Based on Hamilton principle and modal assumption, the complicated vibration of the piezoelectric resonator is simplified as a simple vibration of a single degree of freedom system, and its electromechanical dynamic model is establation force described by a force factor. The model is proved to be accurate by calculation and experiment analysis through an example, and can be used to the earlier design,the proper working frequency's choice, and parametric analysis for ultrasonic motor as well as the simulation model's establishment and the optimization of dynamic characteristic further more.