Abstract: In this paper on the basis of summing up the researching results of predecessors, synthesizing their method of advantages （especially Morgenstern-Price's and Pan Jia-zheng's works）, it has been proved the theorem of maximum value of safety coefficient of the given slip surface and established the Fuzzy Function and the Fuzzy restricted conditions to suit to the concept of traditional safety coefficient and minimum safety coefficient. It has been put forward the concept of Fuzzy Solution Set of safety coefficient and minimum Fuzzy Solution Set, and defined the mathematical model of Strip Partition Method as the problem of seeking the minimum Fuzzy Solution Set being implicitly described functional in basic equations under the boundary condition and Fuzzy restricted condition. On the basis of establishing unknown function spacs Rm, slip surface space Rn and their feasible region and generalized coordinates, it has been put forward FT calculating form which is stable, convenient and possesses the function of reducing unknown functional dimensions,and thereby an optimized numerical calculation method-double complex method hasbeen established. According to the engineering calculating results, a "Engineering Method" is recommended for the use in engineering. The minimum Fuzzy Solution Set evaluated from it can be satisfied the calculating precision required by the standard sufficiently.