RSM　is　used　to　establish　models　in　shape　optimization　for　two-dimension　continuum　by　solving　sensitivity　of　constraint　conditions　approximately,　that　is,　complicated　response　function　in　terms　of　design　variables　can　be　fitted　according　to　the　responses　of　structural　analysis.　The　error　between　the　fitting　value　and　the　structural　response　value　at　the　design　point　will　affect　precision　of　optimization.　This　paper　provides　an　improved　method　that　fitting　accurately　at　the　central　design　point.　The　examples　demonstrate　that　it　is　a　feasible　and　efficient　method.