In the present paper, an analytic model of the planetary gear transmission for the 1.5MW wind turbine generator is established by using lump mass method, in which the linear spring is adopted to simulate the gear tooth meshing under the effect of external load. After that the differential equation of motion and the calculation method of dynamic load coefficient for each gear pair of planetary gear are derived. Based on the above conditions, the paper analyses the influences of various external load on the system. The following conclusion are drawn that the meshing frequency is to be enlargement on the increase of stable wind load, meanwhile, the energy of meshing frequency component is highest; As a result, the primary influence factor of whole system changes from the stiffness excitation to the load excitation; In addition the dynamic load component becomes various, with the low frequency band zone and side band frequency arising. Regarding extreme gust with changeable intensity, the external load undergoes instantaneous change, which leading to a surge of the dynamic load and its coefficient; Regarding extreme gust with changing direction, the meshing force between X-direction and Y-direction appears phase angle, besides, external meshing force along three planetary gears exists phase difference, resulting in that the growth ratio of dynamic load of system is to be once more than gust load and load carrying between gear teeth becomes larger.