To　study　the　design　method　of　cable　sectional　area　of　prestress-braced　steel　frame　structure　system,　lateral　stiffness　of　this　structure　system　was　analyzed　theoretically　in　this　paper.　Formula　of　lateral　stiffness　of　this　structure　was　derived.　The　formula　shows　that　lateral　stiffness　of　this　structure　is　related　to　not　only　the　sectional　properties　and　material　properties　of　members,　but　also　angle　between　cable　and　horizontal　plane.　Lateral　stiffness　increases　with　the　increase　of　story　drift　gradually.　Based　on　the　formula　of　lateral　stiffness,　design　method　and　formula　of　cable　sectional　area　of　this　structure　was　proposed.　Practicability　of　this　design　method　and　formula　was　proved　through　an　example,　the　results　concluded　that　the　prestress-braced　steel　frame　structure　which　is　designed　by　using　this　formula　and　method　can　make　sure　that　the　story　drift　of　structure　meets　code　requirements　under　designed　external　loads.　©,　2015,　Beijing　University　of　Technology.　All　right　reserved.