Abstract: Because of the complicated structure of a planetary transmission system, the effect of gear damage on the dynamic characteristics is not easy to assess, which has become a main obstacle to the fault diagnosis of such a system. Based on the precondition that the inner ring gear is fixed, this paper studied the stiffness synthesis method of healthy NGW21 planetary gearbox and the stiffness degradation behaviors induced by gear surface defect. The meshing stiffness distribution curves of planet gear when engaging with sun gear and ring gear separately were obtained by potential energy principle, then the meshing stiffness synthesis method of multi-point meshing condition was established based on the inner kinetic relationship of the gearbox and phase relationship between the planets. For the case of sun gear and ring gear with crack of different depths, the meshing stiffness distribution of cracked gear meshing with planetary gear was discussed, and finally the stiffness synthesis of the internal and external meshing of the same planet gear was completed. The analysis results show that the crack of the sun gear has more obvious effects on stiffness degradation behaviors and will have significant influence on the system characteristics.