Preload force is a key factor for ensuring the frictional drive of the stator and rotor of an ultrasonic motor. In this paper, the influence of the preload force on the proportion of the contact antenna and driving area is first analyzed through simulation. The velocity fluctuation law of different preload forces is tested experimentally. The velocity and efficiency surfaces under different preload and torque are measured and plotted. With the help of the encoder and film temperature sensor, the changing process of the speed and stator interface temperature of the motor under different preload force was obtained. Based on the above analytical results, a preload force optimization criterion for different applications is proposed, and the ideal working range of the preload force is determined. With the growth of preload force, the contact area is synchronized with the drive area, while the speed of the driving area decreases and the speed fluctuation of the motor is somewhat reduced, the maximum efficiency of the motor occurs at a larger torque. Moreover, the highest rate of mechanical efficiency is at a certain preload force, temperature rise of the interface increases, and surface temperature rise fluctuates. According to the analysis results, the proposed optimization criterion determined that the ideal working preload force range for the TRUM60A ultrasonic motor is 260-320 N. The preload force in this range enables the ultrasonic motor to meet the requirements of low speed stability and small temperature rise, and achieve the ideal range of braking torque and mechanical efficiency.