Using　the　long　span　and　heavy　load　crossbeam　of　some　machine　tools　plant＇s　heavy-type　numerical　control　milling　planer　as　a　subject,　this　paper　builds　a　model　and　force　analysis　through　the　finite　element　software.　The　force　distribution　and　deformation　condition　of　the　long　span　motion　girder　under　the　actual　working　condition　are　also　studied.　The　describing　method　and　establishing　process　of　the　long　span　load　curve,　which　considers　the　strolling　board　superposition　effect,　are　built　by　contacting　analysis　and　restraining　analysis.　The　tool　tip＇s　trajectory　deformation　is　obtained.　The　research　provides　a　sufficient　theory　basis　and　simulation　method　for　improving　the　crossbeam　structure　and　obtaining　a　perfect　arch　in　advance.