Abstract: Oscillation deformation and violent impact between assemblies of fast reactors under seismic excitation directly influence the insertability of control rods, the structural strength and the deformation reactivity of reactor assemblies. The dynamic response of multi-assemblies during seismic condition should be calculated for fast reactor safety. Based on classic arrangement and assemblies structure of the sodium fast reactor core, a three-dimensional multi-assemblies seismic analysis model has been established by utilizing implicit Newmark method in order to solve the non-linear multi-point and multi-angle impact problems. An effective circulating method for impact judgment focusing on multi-ring equilateral triangular layout of reactor core assemblies has been proposed. Testing and validation of the three-dimensional computation model by conducting time historic calculation showed that the present model was better than international similar programs in both accuracy and conservatism, which established a theoretical foundation for engineering application of the independent software.