A calculation model for slotted belleville spring based on the conical shell theory is presented to solve the problem of preloading in the small series of ultrsonic motors. Based on the conical shell theory, the mechanical model for slotted belleville spring is established under the condition of axisymmetric loading and sliding support. The calculation method for the model is studied based on the theory of modern computing methods. A comparative analysis test is made between the experimental data and the results of various computing theories. The results show the model based on the conical shell theory has a higher precision than other methods and the relative error is within ±3.5%. Based on the results of parameter-sensitivity analysis testing for slotted belleville spring, the optimal height-thickness ratio is obtained with the range of 1.5 to 1.8 and the design solution for slotted belleville spring is presented.