Localized defects in operating planetary gears would cause abnormal gears meshing and impact. Correspondingly, the characteristic frequency would be changed. In this paper, a rigid-flexible coupled model for planetary gear trains was built using ADAMS and ANSYS. The frequency domain curve of meshing force was calculated by FFT. At this point, the characteristic frequencies of the defect-free planetary gear trains and both the planetary gear trains and sun gears with localized defects were analyzed. The simulation result is consistent with the theoretical results which indicated that the proposed model of virtual prototype machines is reliable: Respectively, the characteristic frequencies of the defects-free planetary gear trains, the planetary gear trains and sun gears with localized defects are 265.34Hz, 17.67Hz and 36.13Hz. this conclusion could be applied in identification of localized defects in planetary gears and sun gears in gear trains. Furthermore, it provides guidance for research on reliability and vibration performance of planetary gear trains.