The drop law and drop time of control rods have considerable significance for the safety analysis of nuclear power plants under accident conditions. In this paper, a rigid-flexible coupling dynamic model of control rod and guide channel is established for a control rod assembly in a supercritical reactor based on multibody dynamics theory. The corresponding relationship between fluid resistance and control rod dropping velocity is obtained by fluid-structure interaction simulation data, and an approximate formula of fluid resistance is given. The fluid resistance of the control rod at different velocities can be calculated efficiently using this approximate formula. Then Hertz contact theory and Coulomb friction theory are used to calculate the contact force and friction force respectively. Drop time and contact force are obtained by the dynamic simulation of dropping process. The correctness of multibody dynamic model is verified by comparison with the results of finite element model, and the influence of the deformations of control rod and guide channel on dropping process is analyzed. Dynamic simulation analysis of control rod dropping process can be efficiently carried out by the multibody dynamics modeling method presented in this paper, which has important reference significance for the design of control rods and guide channels.