Heavy　duty　deflection　angle　milling　head　is　the　core　functional　unit　of　heavy-duty　gantry　CNC　machine　tools.　Stiffness　of　swing　chain　system　is　one　of　the　important　factors　that　determines　accuracy　of　heavy　duty　deflection　angle　milling　head.　In　order　to　improve　dynamic　performance　and　mechanical　precision,　the　gear　pair　and　support　system　of　the　swing　chain　are　studied　in　the　paper.　Based　on　the　deformation　compatibility　condition　and　the　superposition　theorem,　the　torsional　deformation　and　stiffness　of　each　part　are　analyzed,　and　considered　the　deformation　of　workpieces　and　supporting　system　and　the　interaction　of　them.　The　vulnerable　segment　is　found　out　in　the　paper,　which　is　valuable　to　amend　and　improve　the　design　of　milling　head.　At　last,　the　curves　of　the　relation　between　the　swing　chain　stiffness　and　the　gear　parameters　and　bearing　preload　are　charted,　which　provide　the　theory　and　technical　support　for　digital　design,　optimization　of　the　milling　head　and　the　basis　for　the　similar　driving　system　stiffness　calculation.