It has significant theoretical and practical meaning to propose a method of unsaturated soil slope stability analysis accounting for seepage, deformation and stability simultaneously. Firstly, coupled analysis of transient unsaturated seepage-stress is achieved using the double-stress variables theory of Fredlund. Comparative analysis with pore pressure field is done by the traditional uncoupled method. Variation process of phreatic lines are close to each other, and the maximum relative error of pore pressure is 8.8% at the bottom of embankment; thus the reliability of coupled analysis results are verified. Secondly, it is possible to consider the matric suction term as a part of the cohesion of the soil, and the spatial distribution law of strength parameters of the slope is obtained. Analyzing program of vector sum method of unsaturated soil slope stability analysis subjected to transient seepage based on Matlab has been developed. Finally, taking the influence of drawdown of reservoir water level on slopes stability of embankment for example, comparative analysis between traditional method and proposed method indicates that potential slip surfaces are similar and the difference of safety factors are less than 0.096; thus the rationality of the method proposed in this paper is verified.