In　this　paper,　the　geometric　error　modeling　and　sensitivity　analysis　method　was　established　to　improve　the　compound　machining　accuracy　of　NC　machine　tools.　Use　CHD-25　type　9-axis　5　linkage　turning-milling　compound　NC　machine　tool　as　a　research　object,　based　on　the　multi-body　system　theory　(MBS),　the　geometric　error　modeling　method　of　the　machine　tool　was　developed.　The　model　involves　37　items　of　geometrical　errors.　By　calculating　and　analyzing　the　error　sensitivity　coefficient,　the　key　geometric　errors　that　affect　the　machining　precision　are　identified　finally,　which　provides　an　effective　theoretical　basis　for　the　design　of　compound　CNC　machine　tools.