In order to improve the output performance of the rotating traveling wave ultrasonic motor and study on the optimal prepressures under different loads, the theoretical calculation and experimental verification on TRUM70H ultrasonic motor were carried out. The dynamic model of ultrasonic motor is established, and the influence of prepressure on the mechanical properties of ultrasonic motor is analyzed. Taking the TRUM70H ultrasonic motor as the object of study, the relationship between the optimum prepressure and the load torque is analyzed. A multifunctional testing platform for ultrasonic motor is built to verify the theoretical derivation results, and the reason for the error is also pointed out. According to the conclusion, the prepressure should be selected according to the load when the ultrasonic motors are assembled. The results show that there is a coupling relationship between the prepressure and the load torque on the speed of the ultrasonic motor. In the effective working range of the ultrasonic motor, the optimum prepressure is approximately linearly positively correlated with the load torque. When the prepressure is set according to the actual load demand during assembly, the output efficiency of the ultrasonic motor can be improved.