Abstract: Seismic analysis of fast reactor core assembly is one of primary requirements for reactor safety assessment. Compared to conducting reactor seismic tests, analyzing by codes is more efficient with lower cost, the reason of which a fast reactor core seismic theory has been built up and a specific code for seismic analysis has been developed. The process of constructing seismic model is as follows. Firstly, fast reactor core assemblies are simplified as Euler beams and made use of GUYAN method to increase computation efficiency by freedom reduction. Then the first and second order frequency and damping ratio are utilized to calculate parameters of Rayleigh damping, a kind of structural damping, which connects with mass and stiffness matrixes. Finally, the collision behavior between assemblies is modeled by diagonal and non-diagonal term in stiffness and damping matrixes. The diagonal term presents influence of impact acting on the moving solid itself while the nondiagonal term presents influence of impact acting on the solid that impacts by the moving solid. The diagonal and non-diagonal term, which are named as impact stiffness in stiffness matrix and impact damping in damping matrix, are equal and opposite. Moreover, there are usually two ways to obtain impact stiffness: measurement in shock tests and simulation by finite element method, and the latter one was adopted in this paper. Through applying forces on faces of virtual assemblies modeled three-dimensionally in ANSYS, the extrusion stiffness is feasible to obtain. When the section area and structural strength of assemblies that mutually collided are equal, it can be deducted that the impact stiffness is equal to extrusion stiffness. And the impact damping derives from impact stiffness. During the procedure mentioned above, each term in reactor core assemblies dynamic equation was defined and deducted clearly, then the construction of the fast reactor core seismic model was completed. Meanwhile, an appropriate method was needed for solving the dynamic equation time-historically. Due to the stability, Newmark method was employed as the solution algorithm. Several standard examples and real test were used for the validation of the modeling process and algorithm. Example No.1 was a cantilever beam, and natural frequencies of homogeneous cantilever beam with constant section could be solved analytically. Example No.2 was a real vibrating fuel assembly of CEFR which was simulated by ANSYS. Finally, the real test, conducted by IAEA, contained 19 different assemblies of RAPSODIE reactor vibrating in an array. The assembly beam parameters and seismic excitation data from these examples and real test were input in the seismic model, and the calculation results agreed well with the analytical solution in example No.1, the simulation result of ANSYS in example No.2 and test result in real test conducted by IAEA. The comparison reveals the correctness and availability of this fast reactor core seismic model and algorithm, which prepares for developing the autonomous whole-core assemblies seismic analysis code. In the next stage the fluid structure coupling effect of liquid on the motion of assemblies will be further considered, and the two-dimensional model will be extended to three-dimensional one for expanding the scope of application and improving the calculate precision.