Machining　accuracy　has　always　been　a　key　problem　to　machine　tool.　Machine　errors　are　caused　by　a　variety　of　error　factors　among　which　the　geometric　error　is　a　key　element.　An　accuracy　model　of　the　3-axis　CNC　machine　tool　is　established　in　this　paper　with　multi-body　system　(MBS).And　based　on　the　accuracy　model,　the　mathematical　model　of　roundness　error　also　built　up.　At　last,　the　influence　of　CNC　machine　tool＇s　turntable　angle　error　to　roundness　error　of　machined　hole　is　analyzed.