Gearbox is the core component of wind power train，the operating of which directly affects the normal operation of wind power system, therefore, it is of great significance to carry out the dynamic characteristics of wind power gear box. The finite element model of coupling system composed of gear-rotor-bearing-box is established. By using Lanczos method，natural frequencies and vibration models are obtained. According to the method of experimental modal analysis，the MW wind power gearbox is tested by using SIMO method, the result of which agrees with theoretical calculation. Both of them verify the reliability of finite modal analysis of MW wind power gearbox.