To　study　the　design　method　of　sectional　area　and　initial　tension　of　wire　rope　brace　of　the　prestressed　braced　steel　moment　frame　structure　system,　theoretical　analysis　of　this　structure　system　is　conducted　in　this　paper.　The　lateral　stiffness　formula　is　derived.　It　reveals　the　lateral　stiffness　is　related　to　the　lateral　stiffness　of　bare　steel　moment　frame,　story　height,　the　distance　between　column　and　lower　end　of　brace,　story　drift,　material　properties　and　sectional　properties　of　wire　rope.　The　lateral　stiffness　increases　with　the　growth　of　story　drift　and　the　relationship　curve　is　a　concave　shape.　It　is　presented　the　initial　prestress　degree　design　formula　and　method　in　light　of　the　criterion　for　determining　initial　prestress　degree.　The　story　drift　decreases　with　the　growth　of　wire　rope　sectional　area　and　the　relationship　curve　is　a　concave　shape,　in　terms　of　this,　a　wire　rope　sectional　area　design　formula　and　method　are　proposed.　The　validation　of　the　proposed　design　formula　and　method　of　wipe　rope　brace　is　proved　by　an　example　analyzed　using　finite　element　software　package　ABAQUS.　©　Zhang　et　al.