Abstract: An improved immune genetic algorithm has been put forward in this paper. It is based onthe thoughts of vaccine selection, vaccination, immune memory, gene affinity mutation, gene recombination. On the basis of the algorithm, a search method for the most dangerous slip surface of soil slopes has been studied. The method is further combined with Monte-carlo stochastic simulated technique. The method has been used in the soil slope reliability analysis. An actual case study is given. The calculating result for the actual case is in accordance with the fact. The study verifies the validity of the algorithm and the correctness of the method.