By analyzing the ex-core neutron noise signal， the vibration frequency of the core support barrel （CSB） is identified and its change is tracked. It is found that during the operation of the pressurized water reactor （PWR） nuclear power plant， the vibration frequency of the CSB beam mode during the fuel cycle shows a downward trend. However， after the replacement of the nuclear fuel， the vibration frequency of the CSB beam mode at the beginning of the new fuel cycle basically returns to the initial value of the previous fuel cycle. The static analysis of the CSB is carried out to determine the potential influencing factors on the modal. The assembly model of PWR reactor internals is established and the friction contact method is adopted to comprehensively deal with the constraints of the CSB flange. The fluid-structure coupling method is used to map the pressure of the coolant on the CSB to the CSB structure. Finally， the prestressed mode analysis of the CSB is carried out by the finite element method， and the causes for the drop of the vibration frequency of the CSB beam mode are obtained. The results show that the drop of the vibration frequency is caused by the stiffness degradation of the fuel assembly hold-down springs. As the stiffness of the fuel assembly hold-down springs decreases， the axial preload is insufficient to offset the coolant force， resulting in the deformation of the CSB flange， which causes the reduction of the contact stiffness， so that the modal frequency drops.