Sodium-cooled fast reactor is one of the important reactors of the fourth-generation advanced nuclear power technology. Since the pool-type fast reactor has large size and thin wall that its rigidity is usually low. Therefore, the seismic design, analysis and verification of fast reactors are the key concerns of fast reactor safety evaluation. The fluid structure interaction effects of many special structures with narrow gaps under seismic loading need to be more accurately evaluated. At present, the method of added mass in the narrow gap fluid domain and the added mass formula of the infinitely long flexible coaxial cylinder given by the pressure field wave equation is not suitable for direct use. Based on the finite element analysis method, the frequency of different narrow gap annular cylinders such as coaxial three-layer cylinders in different modes is studied. According to the vibration formula, the added mass of narrow gap fluid domain is obtained, and the main pump support in the fast reactor is obtained. And explore the effect of the height of the fluid in the shell gap on the added mass of the shell. The design and inspection of narrow gap watershed structures such as cylinders and stack bodies have reference value.