The inner hole of long axis workpieces is usually machined using a large-diameter boring bar.Due to the large aspect ratio of the boring bar,the overall stiffness will be greatly reduced,and the boring bar will vibrate during cutting.To optimize the passive damping boring bar structure,a new type of boring bar with variable damping dynamic damper was designed,and its basic parameters were calculated.Based on a two degree of freedom dynamic model,the vibration reduction mechanism of a boring bar with a dynamic damper was analyzed,and changing the damping parameters could significantly improve the vibration reduction effect.By axially moving the damping block inside the boring bar,under the action of the inclined surface inside the boring bar,the rubber ring was compressed to change the damping parameters.Then the specific structure and size of the boring bar were determined.When the excitation frequency is greater than 325 Hz,the proposed boring bar can achieve vibration reduction by changing the damping parameters.In actual machining,the boring bar should be kept above this frequency for cutting as much as possible.Finally,a simulation analysis was conducted on the relationship between rubber damping and axial movement of the damping block.The results show that as the distance of the damping block movement increases,the damping ratio also increases,indicating that axial movement can change the damping value of the vibration damping system.