The　superior　performance　of　machining　large　parts　with　the　free-form　surface　is　demonstrated　by　a　five-axis　gantry　machine　tool　composed　of　a　gantry　mainframe　and　a　milling　head.　A　turntable　is　used　to　adjust　the　position　of　the　workpiece　rather　than　a　linear　guideway　when　machining　impellers　or　integral　turbines.　Furthermore,　collisions　between　adjacent　blades　are　almost　unavoidable　in　the　machining　region,　which　is　relatively　narrow　for　milling　heads.　The　majority　of　existing　methods　attempt　to　solve　the　problem　by　changing　the　tool＇s　tilt　angle.　However,　when　an　end-mill　cutter　is　employed,　this　method　will　deteriorate　surface　quality　and　reduce　efficiency.　On　the　condition　that　the　milling　head　has　three　rotary　axes　(TRA),　a　novel　NC　code　generation　method　with　a　fixed　tool　orientation　vector　for　narrow　space　surfaces　is　proposed　in　this　paper.　When　the　geometrical　model,　considered　a　supplementary　condition,　is　combined　with　the　kinematics　model,　an　inverse　kinematics　solution　to　a　general　multi-axis　gantry　machine　tool　with　a　TRA　milling　head　is　obtained.　The　problem　of　multi-axis　coupling　in　the　inverse　kinematics　model　is　solved　by　combining　the　iteration　method　and　geometrical　model.　In　the　narrow　space　for　a　milling　head,　the　redundant　degree　of　freedom,　provided　by　a　machine　tool　equipped　with　a　TRA　milling　head,　enables　adjusting　the　posture　of　the　milling　head　without　changing　the　tool　orientation　vector.　Feature　points　are　extracted　by　preprocessing　points　selected　from　machined　parts　and　the　TRA　milling　head　to　choose　the　feasible　posture　and　simulate　the　machining　process.　Collisions　in　the　narrow　space　are　successfully　avoided　throughout　the　cutting　process.　The　NC　code　generation　method　is　verified　on　CAM　software　and　cut　experiments,　proven　effective　in　this　paper.