Abstract: The flow-induced vibration（FIV） of the core barrel is very common, and the characteristics of the FIV is important to the security of the nuclear reactor. By monitoring the vibration characteristics of the core barrel, the natural frequencies of the core barrel would decrease, as the nuclear reactor is running, especially for the fundamental natural frequency of the core barrel. It is difficult to calculate this problem with the conventional method. In order to solve this problem, a new method to simulate loose-joined flange between the top of the core support barrel（CSB） and the head of the vessel was presented in this paper. The dynamical finite element model of the structure of the bore barrel was obtained and the fundamental frequencies of the core barrel were calculated, including 8 situations, such as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% loose-joined flange between the top of CSB and the head of the vessel. The comparison between the calculated results and the experimental ones show that the method developed in this paper is reasonable and feasible.