In order to study the dynamic problems of wind turbine tower system, such as wind-resistant and anti-seismic, modal analysis of a 1.2 MW wind turbine is carried out based on finite element numerical simulation to research the vibration characteristics of each mode. By changing dimension variables, such as blade length, the height and thickness of tower, the influence of variables on the natural vibration characteristics of wind turbine system is also studied. Through the function fitting of many groups of numerical simulation results for conditions, the fitting formula with high confidence is derived, which ascertains the function relation between dimension variables and natural vibration frequency of wind turbine system,Thus a theoretical basis for structure dynamic analysis of wind turbine system is provided to project practice for references.