Abstract: Boundary condition of vibration characteristics both in air and still water about the empty core barrel of Qinshan Nuclear Plant 600 MW reactor and CAP 1400 type reactor were investigated. The structure of core barrel was modeled by solid element, and the fluid structure was simulated by fluid element. The close effect between the flange of core barrel and support step of reactor pressure vessel was assumed with three types of boundary conditions: weak close, medium close and strong close. At the same time, the friction between the flange of core barrel and support step of reactor pressure vessel was assumed with three types of boundary conditions: weak friction, medium friction and strong friction. The research shows that: The boundary condition of core barrel in still water is much stronger than that in air for Qinshan Nuclear Plant 600 MW reactor, but the boundary condition of core barrel both in still water and in air is the same for CAP 1400 type reactor. The preload of hold down spring and friction of flange have a certain effect on the core barrel frequency.