The rigid flexible coupling dynamics of a rectangular functionally graded thin plate undergoing large overall motion is investigated.Based on continuum medium mechanics, The governing equations of motion are established using assumed mode method and Lagrange′s equations, for the rectangular functionally graded thin plates undergoing large overall motions. The second order coupling deformation variable, which is ignored in traditional method, is considered in this paper. The dynamics of cantilever plates with different functionally graded coefficiernt undergoing rotation are simulated and the deformation results of first order approximation model are compared with those of traditional zero order appoximation model. It is shown that, as the rotating velocity increase,the traditional model divergences while the first order model converges.The first order model describes the dynamic behavior of the system better. The effects of the functionally graded index on the deformation of plates are studied. Studies have shown that, with the increases of the functionally graded index, the greater the lateral deformationthe of plate. By solving the natural frequencies of rotating FGM plate under a constant speed,the impact of of changes in the material composition of FGM plates on the plate vibration characteristics is further studied.