In this paper studied a rotary system with a nonlinearity, the equations of motion of which is one of the types of the Duffing’s equations with multidegree-of-freedom, and also consider the advantages of using the elliptic function method to solve problems of this type. The system studied in this paper is also distinguished by the fact that in addition to the rotor vibrations in elastic supports with a nonlinearity, the vibrations of the foundation are also taken into account. The article presents a comparative analysis of the numerical Runge-Kutta-Fehlberg's 4-order method with an error estimate of 5-order, the approximate analytical Van der Pol’s method and the elliptic function method proposed by the authors by comparing the obtained equations of motion of the system, as well as by comparing the constructed frequency response characteristic. From the results obtained, it follows that the method proposed by the authors can serve as a more accurate, general case of previously used approximate methods.