Abstract: It is discussed the optimization design of prestressed cable supported steel truss with its design variables including continuous cables' pretension force and discrete cross section size.A optimization mathematical model is established in such a form that the member's stress and structural mass are constrain conditions and the minimum structural strain energy is the objective function.The cables' pretension force can be determined by minimizing all member bars strain energy when prestress and external load apply together.Each member's relative difference quotient which mass variations divide energy variations caused by the member's size change is calculated to determine searching direction.Evolutionary structural optimization is adopt to modify member cross section size gradually untill cables' pretension force and members' cross section size reach optimal by iteration.Result of examples in the paper coincides with that of prestress steel structures theory,the method in this paper can provide optimum cables' pretension force and formed steel for the prestressed steel structure design.