Abstract: In order to calculate the dynamic response of the control rod drive mechanism（CRDM） under the stepping loads, a three-dimensional CRDM finite element model is established based on the theory of flexible multi-body dynamics, in which the coupling of rigid-body motion and elastic deformation is considered. The modal reduction method is used to reduce the number of deformation variables in the equations, so that such model is more efficient than the nonlinear finite element method in calculating the motion curve, impact load and stress response. In addition, the present model is more accurate than the conventional finite element method using the rod element.Not only the motion curve, but also the stress distribution of each component can be accurately obtained to analyze the weak position.