The governing equations of seepage analysis are derived by the law of conservation of mass, and this type of FEM issue is solved using fully implicit backward difference format. The method of FEM strength reduction of unsaturated soil is employed in construction stability analysis of the high slope. The safety factors and sliding surfaces of every construction phase on the effect of rainfall infiltration are obtained, and useful conclusions are drawn by comparing with the results of no rainfall infiltration. Furthermore, on the basis of the research results, advices on the reinforcement are proposed. The ideas and methods proposed may serve as a reference for the design and construction of similar projects.