In this paper, a quasizero stiffness disc spring rotor which maintains the prepressure constant under the external environment is designed against the background that the normal working prepressure of the ultrasonic motor changes due to longterm storage and high impact environment. The parametric finite element model is established by referring to the design idea of the slotted disc spring. The main design variables are selected by sensitivity analysis. The longest objective function of the quasizero stiffness section near the selected working prepressure is established. The finite element numerical model is used for the optimized design of the parameters of the disc spring rotor. The results show that the length of the quasizero stiffness section of the optimized disc spring rotor reaches 0.6 mm near the normal working prepressure, and the strength requirement is met under the impact environment. Finally, the experiment verifies that the ultrasonic motor with the disc spring rotor has good output characteristic.