In　the　factory,　there　are　many　hollow　shafts　with　thin　wall　need　to　be　machined　using　boring　operations.　When　machining　the　workpiece　with　thin　wall　the　relationship　between　the　dynamics　of　the　bar　and　the　workpiece　has　a　strong　and　complex　influence　on　the　machining　stability.　The　reason　is　that　the　stiffness　of　workpiece　is　at　the　same　magnitude　as　the　boring　bar.　The　machining　stability　is　sensitive　to　the　minute　change　of　the　relationship　between　the　dynamics　of　the　bar　and　the　workpiece.　At　the　same　time,　it　is　difficult　to　analyze　and　improve　the　machining　stability　because　the　dynamics　of　the　workpiece　has　a　significant　change　along　with　the　cutting　process.　For　the　boring　chatter　avoidance,　this　paper　gives　a　mathematic　model　depicting　the　chatter　mechanism　for　boring　the　thin　walled　workpiece.　By　utilizing　numeric　simulation　method　in　the　time　domain,　it　is　analyzed　that　how　the　relationship　between　the　dynamics　of　the　bar　and　the　workpiece　affects　the　boring　stability.