Abstract: Based on the slip line field theory(SLFT), a new instability criterion of strength reduction method with double re-duction factors for homogeneous slopes was proposed. It is judged that the slope is in the limit equilibrium state when the slope contour under limit equilibrium state(abbreviated as the critical slope contour) calculated by the SLFT intersects with the slope line at the toe. The critical curve was constructed in the plane of the inverse of the double reduction factors, and the comprehensive safety factor was derived. The slope calculation results show that three relative position relationships between the initial point and the critical curve, i.e., the point is above, on, or below the critical curve corresponding to three types of safety factors, i.e., FS>1, FS=1 or FS<1. The proposed instability criterion is suitable for calculation of the reduction proportionality factor and the comprehensive safety factor, and realizes the objective quantification of the instability criterion.