Abstract: The liquid metal reactor has a narrow fluid gap between the reactor vessel and the thermal shield, which is a small aspect ratio coaxial flexible cylindrical shell containing a narrow gap. The fluid-structure interaction in the narrow gap produces fluid-structure interaction effects that must be considered in the seismic analysis. The study mainly focuses on cylindrical shells with relatively large aspect ratios and large fluid gaps, and there are few studies on coaxial flexible shells with small aspect ratios and very small fluid gaps. Three different sizes of narrow gaps were designed with reference to the structure of liquid metal reactor. The vibration experiment of small aspect ratio coaxial cylindrical shell model with narrow gap conditions was carried out. The modal frequencies and main vibration modes were obtained. Then, the modal analysis was performed using the finite element method, and the experimental and finite element results were compared. Finally, the added mass of the model at different gap sizes were calculated to verify the applicability of the added mass calculation method, and the effect of different gap sizes on the fluid-structure interaction characteristics was investigated. This study can provide input for the seismic design of the similar coaxial flexible cylindrical shell structure containing a narrow gap with a small aspect ratio, and is of great significance for the seismic analysis of liquid metal reactors.