Abstract: Nuclear power plants requires the spent fuel assembly grippers to be able to withstand safety shutdown earthquake(SSE), and seismic response of grabbing process is the important basis for evaluating its safety. To analyze the seismic response of grabbing process accurately, according to the structure and working process of the spent fuel assembly gripper, the mixed-double-pendulum model of the spent fuel assembly gripper was established, and the differential equation of motion was derived. The Runge-Kutta method is used to solve the response of the gripper under seismic load. The dynamic loads on the wire rope and gripper are further obtained according to the dynamic theory, and the results are analyzed. This method offers reference for the accurate seismic design and comprehensive evaluation of the similar structure.