Cross-rail　is　a　key　component　of　support　structure　of　the　machine,　whose　mechanical　properties　have　direct　impact　on　the　accuracy　and　dynamic　performance　of　the　machine.　This　paper　studied　a　bridge　type　milling　composite　extrusion　CNC　machine　whose　cross-rail　welded　together　by　a　rib　structure,　therefore,　well-designed　rib　structure　is　particularly　necessary.　In　this　paper,　the　finite　element　method　for　analyzing　the　cross-rails　of　four　different　rib　structures,　static　deformation　and　natural　frequency,　and　the　deformation　of　impact　on　the　process　of　start　and　stop　in　three　performances　are　adopted　to　obtain　better　overall　performance　rib　structure　to　meet　the　requirements　of　the　mechanical　properties　of　the　machine.