Abstract: It is necessary to consider the fluid-solid coupling effect due to the interaction between coolant and core assembly when analyzing the seismic performance of the FBR core assembly. The added damping was treated mostly as a constant in previous researches. In fact, the effect on assemblies from the coolant depends strongly on the gap between the core assemblies, and the damping should be considered as a variable. In order to simulate the vibration of the core assembly more accurately, the methodology for the seismic analysis of FBR core assemblies using variable added damping was studied. In this paper, the seismic analysis model of one single row of core assemblies (5 assemblies) of FBR was established. By comparing the two kinds of added damping models, the constant and variable ones respectively, the results show that the seismic analysis of the core assemblies with variable added damping is feasible and effective. Meanwhile, the simulation method used in this paper can obtain more precise approximation of the vibration of the core assembly and lays the foundation for more realistically simulating the seismic response of reactor core assemblies. It also helps to reduce the conservative margin of the structural design and is meaningful in engineering application.