Finite element method is used in the dynamic model which takes whirling of gear-carrying shaft into consideration to analyses the natural characteristics of a one-stage gear system. The analysis results of the dynamic model are then compared to those of finite element software and experiment. It shows that they reach agreement in term of natural frequency. Therefore the dynamic model with finite element method is proved effective. With the theory of stability in geared rotor system,some of the parameters of high speed dynamic characteristics and responses such as critical speed are gained. The dynamic model of a one-stage gear system provides a survey to the design and the stability analysis of high speed gear system.