The pre-pressure of the ultrasonic motor will be easily changed by external influence, which affects the stability of its operation. For the application of ultrasonic motor under overload，a new structure must be designed to keep its pre-pressure basically constant. Based on the PMR40 type ultrasonic motor, a slotted disc spring is designed in this work as a buffer to improve the structure of the motor. The impact dynamics model of the whole motor is established under maximal impact acceleration of 1 000 g~5 000g to analyze the influences of the disc spring on the pre-pressure of the motor. The results indicated that the disc spring had obvious maintenance impact on the pre-pressure of the motor when the maximal axial acceleration is under 1 000g. The variation rate of pre-pressure for the motors without and with the disc spring were 28.1% and 10.0% respectively. While under high overload condition (maximal acceleration ≥ 2 000g), the ability of the disc spring to maintain the pre-pressure of the motor is decreased, but its buffering effect can reduce the change of pre-pressure relatively, so as to maintain the stability of the motor. This work is of important guiding significance for the design of ultrasonic motors suitable for overload.