Cracks are a form of early failure in the planetary gear system of a wind turbine. Aiming to solve problems of crack diagnosis caused by the amplitude and frequency modulation from inevitable gear errors in planetary gear systems, a dynamic model containing various types of manufacturing errors is built and used to consider the influence of cracks on mesh stiffness and transmission error. Using numerical calculations, the sideband of the spectral envelope of the system with cracks on its planet, sun and ring gear are compared with the sideband of a normal system without cracks. The fault characteristic frequencies of the three parts are concluded. The crack fault experiment is carried out on a test bench of a wind turbine gearbox, and its results are helpful for the fault detection and localization of the planetary gear system in wind turbine gear systems.