Machining　accuracy　is　a　very　important　index　to　measure　the　performance　of　NC　machine　tools,　and　how　to　improve　the　machining　accuracy　grabs　the　attention　of　both　manufacturers　and　users.　Since　the　geometric　errors　are　intercoupling　and　are　one　of　the　main　factors　affecting　machining　accuracy　of　machine　tools,　in　this　paper,　a　new　method　for　extracting　key　geometric　errors　based　on　the　screw　theory　and　extended　Fourier　amplitude　sensitivity　test　(EFAST)　method　was　proposed.　First,　screw　theory　was　introduced,　and　the　machining　accuracy　modeling　of　a　three-axis　vertical　machining　center　(VMC)　was　established.　Then,　in　order　to　analyze　the　coupling　effects　between　geometric　errors　by　high-dimensional　nonlinear　model,　EFAST　global　sensitivity　analysis　method　was　adopted　to　help　in　extracting　the　main　geometric　errors.　An　illustration　example　on　a　three-axis　VMC　shows　that　the　proposed　method　is　effective　and　can　be　helpful　to　improve　the　geometric　accuracy　of　machine　tool.