In this paper, the problem of conditional extreme-value of solving structural reliability index is transformed into the problem of non-conditional extreme-value by use of Lagrange Multiplier Method. 本文用Lagrange乘子法把求解结构可靠指标的条件极值问题转化为无条件极值问题,对目前已被应用的迭代公式给出了理论证明;