Two key issues in landslide forecasting are finding the critical slip surface and determining its parameters. Generally, these issues are two parts of one problem. To solve this problem, a new two-phase search method is proposed: a newly proposed inversion method to determine the critical slip surface parameters, using a new bionics algorithm (i.e., particle swarm optimization), and based on the measured slip surface and the critical slip surface-searching process. The proposed method is verified by using two typical numerical examples used by the Association for Computer Aided Design (Australia) and an engineering example of a highway landslide. The results indicate that when the search over a critical slip surface and the inversion of its parameters are performed simultaneously, the calculated factor of safety is in good agreement with that of the actual scenario, whereas the factor of safety computed on the basis of known parameters of a slide body will deviate from that of the physical scenario.