Abstract: 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.