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.