In this paper，a 9-DOF (Degree of Freedom) flight dynamic model of controllable parafoil system is presented. Based on this model，the simulation program is developed to aid the study of the gliding performance and dynamic stability for such a system with a parafoil of 40(sqm). Its results are compared with those of 6-DOF model. The stablitity of relative pitch is analysed in detail，and the methods for its improvement are given.