Critical sliding surface search has been an important research topic in the field of slope stability analysis. The limit equilibrium method is still playing a major role in the design of slope because its concept is simple relatively and it is easy to grasp by engineers. Residual thrust method is one of the limit equilibrium methods proposed by Chinese researchers which is practical and has high accuracy in engineering application

and it has been brought into national regulation. But considering the drawbacks that inter-slice force is too crude and only equilibriums of forces are used without regarding to moment equilibriums

we construct a model which satisfies all equilibrium conditions through iteration of Spencer method. And then combining it with ant colony algorithm

we find out the critical slip surface which has the maximal residual thrust. The model has characteristics of high accuracy and fast convergence. Finally

two cases are calculated and compared with the model that has the minimum safety factor based on the ant colony algorithm too. The results indicate the correctness of this method.