Abstract: The vacuum chamber would vibrate when it was inspired by outside excitation,and the vacuum chamber would have a resonance when the external excitation frequency was close to it's natural frequency.The vacuum chamber serves as a running place for the Particle beam,if the resonance occurs,it will not only affect the air tightness of the vacuum system,but also may cause damage to the structure.In this paper,based on the actual working conditions of thin-walled Dipole-Magnet vacuum chamber,the free vibration mode and the pre-stress mode of the vacuum chamber are analyzed by using finite element software.The conclusion is that the structure natural frequency of the first six order modes is 0 under the free vibration,which belongs to the rigid body mode.Under the prestress,the natural frequency of the first six order modes is in the range of 400-720 Hz,and the first and second order modes are integral modes,whose modal characteristics are mainly concentrated on the thin wall and reinforcement in the middle of the structure,while the 3rd to 6th order modes are local modes,whose modal characteristics are mainly concentrated on the reinforcement in the side of the structure.