The landslides from the instability of slopes are very serious geological hazards. The one key issue to compute the slope stability is the searching of critical slip surface. Generally, the searching of critical slip surface is a very typical complicated continuous optimization problem. To solve this problem very well, firstly, combing the artificial immune system algorithm and evolutionary algorithm with continuous ant colony algorithm, one new bionics algorithm for continuous function optimization which is called immunized continuous ant colony algorithm is proposed, secondly, combing new algorithm with limit equilibrium analysis, one new global optimization algorithm for critical slip surface searching is proposed. At last, through a typical numerical example-ACADS example, this new method is verified. The results show that, using the new algorithm, the searched slip surface will be coincided with the measured slip surface very well, and the stability safety factor will also be agree with the actual situation.