The soil slope stability under unsteady seepage is a complex issue related to both hydraulics and soil mechanics. The unsteady seepage caused by water level drawdown goes against the slope stability of dams and embankments. This paper has carried out model experiment on slope stability under unsteady seepage, studied influence on slope stability caused by seepage flow under different conditions both hydraulics and soil mechanics, and discussed the relationship between water drawdown index k/μv and the slope stability. The results shows： The seepage direction is the key factor. The direction of the seepage flow points to the upstream, the safety factor is smaller in this direction than other directions. The slope failure occurs when the seepage gradient reaches the peak. The result which is not considered seepage direction can not correctly reflect the stability of slopes. The slope stability analysis which needs to be combined with seepage is proposed.