Because planetary gear sets contain many components and have complex transmission structures,their dynamic models are difficult to be established. Besides,the planet gears rotate around not only their own centers,but also the center of the sun gear,and the time-varying distances from gear meshing points to the fixed transducers add the complexity of vibration response. Most of the dynamic models reported in current literature are under healthy conditions,and the time-varying effects of transmission paths are ignored in these models. Aiming at these shortcomings,a new dynamic model of planetary gear sets is established,in which the time-varying mesh stiffness under healthy,cracked and spalling conditions in planetary gear sets is calculated respectively,and the transmission paths are considered. Using this model,gear dynamic responses are obtained and compared with vibration signals captured from a planetary gearbox test rig. The comparison verifies the validity of the dynamic model,and provides a theory basis for health monitoring of planetary gear sets.