The　article　used　the　large　span　and　heavy　load　cross-rail　of　some　machine　tools　plant＇s　heavy-type　numerical　control　milling　planer　for　research　object,　by　using　the　finite　element　software　to　build　the　model　and　force　analysis.　It　also　researched　the　force　distribution　and　deformation　condition　of　the　large　span　motion　girder　under　the　actual　working　condition.　It　considered　the　slide　carriage　superposition　effect　to　build　the　describing　method　and　establishing　process　of　the　large　span　load　curve　by　contacting　analysis　and　restraining　analysis,　and　got　the　knifepoint　trajectory　deformation.　The　research　provided　the　sufficient　theory　basis　and　simulation　method　for　designing　improvement　of　cross-rail　structure　and　got　a　perfect　arch　in　advance.