In　view　of　defects-＇chew　cutting＇　generated　by　the　five-axis　nc　machine　tool　in　the　process　of　machining　the　S　sample,　the　machine　tool　motion　error　model　is　established　based　on　multi-body　system　theory.　After　that,combining　sensitivity　analysis　theory　with　the　research　of　machine　tool　processing　defect　causes.Then　find　out　the　main　error　terms　which　take　great　responsibility　of　machining　defects　and　optimize　related　motion　components,to　meet　accuracy　requirements　before　leaving　the　factory.　©　(2014)　Trans　Tech　Publications,　Switzerland.　All　Rights　Reserved.