As a key component of planetary transmission system, the ring gear plays an important role in load sharing and dynamic load factor of planetary transmission system. With the help of the flexibility of ring gear, the non-uniformity of the transmission torque about each planetary wheel caused by the machining error and installation error of the system components can be significantly compensated. In this paper, the structural characteristics of flexible ring gear are deeply analyzed. The comparison shows that flexible ring gear has better flexibility than the traditional ring gear , which can improve the load sharing and dynamic load factor. But at the same time, there are some disadvantages, such as Unbalanced loading, uneven stress distribution, increasing amplitude of loaded transmission errors and stress concentration of structure. The relevant analysis results can provide theoretical support for the design of flexible ring gear and the improvement of ring gear structure.