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.