Due to the influence of deformation, the load distribution of planetary transmission system is not uniform, and the wave shock is frequent, which is prone to unbalance loading and stress concentration, and the extreme condition of alternating heavy load and strong impact become more and more prominent. The existing methods to improve the performance and prolong the life of planetary systems have little effect on the harsh conditions of alternating heavy loads. In this paper, the floating flexible ring gear is proposed based on the foreign key-type ring gear, which is composed of inside ring and outside ring, the ring gear is a thin-walled structure, and the circumferentially restricted motion and radial slip are realized between the inner ring and outer ring through a convex key, and the clearance is used to realize floating adjustment and overload protection. The strong flexibility of inner ring can realize the unbalance loading of system each branch and absorb kinetic energy to improve the dynamic load. The analysis shows that floating flexible ring gear has improved the load sharing and dynamic load performance significantly compared with the full fixed ring gear and L-shaped ring gear, which can provide theoretical guidance for the ring gear design of planetary transmission system.