Directional simulation method is proposed to calculate the small probability of slope failure. Algorithms of directional simulation and wedge safety factor are introduced and programmed. The direction simulation results and the importance sampling respectively, compare the calculation results with the subset simulation analysis. The results show that the directional simulation method can be used to calculate the small probability of slope failure as the same with subset simulation method. And directional simulation method can get enough accuracy by a small number of sampling, which means more efficiency than the other two methods.