Abstract: Based on the immune algorithm and geometric implication of the reliability index,a global optimization method was put forward to calculate the reliability index.The algorithm simulated the natural process of biological immune systems,and the basic steps of the algorithm included: computing affinity between the B-cells and the antigen and selecting;cloning and mutating of the B-cells and producing of memory cells etc.By this method,reliability of the nonlinear and complex performance function of geotechnical engineering could be obtained without derivation.It was shown by two practical cases that the present method was reliable and accurate in the reliability analysis of geotechnical engineering.