Abstract: The two-step optimum method is adopted to optimize the cross-section size,and the cable pretension force for the object of minimum weight.Considering constraints of stress,displacement and stability,a new optimum design model is established for the object of minimum price based on the above optimum,which accounts for constraints of stability and displacements.Considering kinds of quoted price of steel cable and steel wire rope in the market,the optimum design method for the minimum price is put forward to obtain the cable diameter and thickness.Some examples,such as long span pre-stressed plane truss string structure and normal plane truss string structure are provided to verify the suggested method and conduct economic analysis between pre-stressed long-span plane truss string structure and normal plane truss string structure.