Abstract: By means of the fliud-structure interaction, the dynamical characteristics of the core barrel of nuclear reactors in air and in stillwaterwere investigated using programANSYS. Asolid modelwas used to simulate the core barrel, the fluid structurewas simulated bythe elementof fluid80, thus the effectof added mass of still water on the dynamical characteristics of the core barrel was included. The research results showthat both in air and in water, the first several order nature frequencies of the core barrel are formed frombeam and shell frequencies under its different boundary conditions, and circumferential rotation and axial flexible frequencies are its higher order frequencies. Moreover, the vibrational frequencies of the core barrel in still water are much lower than its corresponding order frequencies in air.