Based　on　the　error　influence　tracing　analysis　for　complex　machine　tool　has　been　carried　out　based　on　＇S＇　shaped　workpiece.　A　method　of　judging　the　error　parameters　which　is　the　biggest　influence　on　machining　defects　is　established.　In　this　method,　the　machining　defects　of　＇S＇　shaped　workpiece　and　shaping　motions　of　complex　machine　tool　are　comprehensively　considered.　Cubic　B-splines　surface　has　been　applied　to　surface　representation.　The　actual　position　of　tool　center　can　be　deduced　by　projecting　the　B-splines　surface　in　its　normal　direction.　The　mapping　relationship　between　the　actual　tool　position　and　the　actual　machining　curve　is　established.　The　machining　defects　generation　model　is　established.　The　error　expression　equation　has　been　deduced.　Those　5　key　parameters　that　have　great　influence　on　processing　defects　are　determined　according　to　the　contribution　which　have　been　computed　using　the　sensitivity　and　measured　values　of　error　parameters.　Experimental　results　demonstrate　that　the　error　of　each　point　is　not　more　than　±1.5　μm　by　comparing　the　five　error　parameters　and　all　parameters　under　the　action　of　at　the　same　time.　The　biggest　errors　which　influence　on　the　machining　defects　are.　©　2016　Journal　of　Mechanical　Engineering.