The cohesion c and internal friction angle ? play different roles in the progressive failure process of the slope, which indicates that the reduction factors k[sub.c] and k[sub.? ] should be different in the calculation. Based on this, the program of double strength reduction factor method was compiled with FISH language, in order to study its application in the rock slopes under different distributions of weak interlayer, and the following conclusions were drawn: (1) the plastic zone calculated by double strength reduction factor method is generally distributed in the weak interlayer, which is basically consistent with the calculation result of the traditional method; (2) the degree to which c and ? play a role is related to the inclination angle of the bottom sliding surface of the unstable block ? . If ? k[sub.? ]? can be adopted when ?