The failure of slope is a progressive process that a special point's strength is over its peak value to fail and then the failure evolves from points to lines and then lines to surfaces. And, the failure surface must correspond with the surface which has the minimum global safety factor. According to the knowledge of optimal principle, the global safety factor will reach its minimum value while all local safety factors on this failure surface being their minimum value. Based on above perceive, a new method to search the slope critical slip surface from points to line and then surface is proposed. And the formula of global safety factor expressed by a series of local safety factors is deduced as well. Also, the reliability of the recommend method to search the critical slip surface is verified with two critical slope cases. Moreover, it is points that the space between points should smaller than 0.04 times of slope length to acquire accurate results.