Abstract: It was important to search the critical slip surface in slope stability analysis.Based on the algorithm suggested by Venanzlo R.Greco,which was extended in this paper,in order to reduce the number of trial slip surfaces which did not meet the condition of geometrical rationality,a condition of geometrical rationality that the slip surface and slope surface had no point of intersection except the two end points of slip surface was added,and the mechanism of movable orientation control and variable degree of freedom of slip surfaces were added too.All the above techniques ensured the accuracy,efficiency and stability of algorithm.The extended algorithm was adopted by the slope stability analysis software ZSlope developed by the authors,and the software could display every real time trial slip surface.