Abstract: Through tests the soil-water characteristic curves and strength parameters of unsaturated soil are determined; and then by way of theoretical analysis a model of seepage-stress coupling model is built. After that, coupling calculation is introduced to analyze the unsteady seepage flow of the unsaturated embankment due to the change of water level and then by using the shear strength reduction FEM to analyze the stability of the unsaturated slope because of the repeated rise and fall of the water level. The results indicate that when the water level rises, the seepage velocity at the bottom of embankment slope decreases, and then increases and grows steady. With the rapid rise of the water level, the global stability of embankment decreases; with adjustment of pore water pressure in seepage field, the safety factor of the slope stability increases to some extent. When the water level falls too quickly, the degree of saturation in clayey soil changes relatively slowly; the sudden fall of the water level accelerates the seepage velocity of embankment, and so the stability of the slope decreases fast. What’s more, about 120 hours after the fall of the water level the embankment enters a dangerous period. In addition, the matric suction of unsaturated soil increases the global stability of the slope and the repeated rise and fall of the water level decreases the global stability of the slope. What is more, the lower the river water level, the more severely the stability of the embankment will be affected

Key words: embankment, unsaturated soil, seepage, stability, strength reduction FEM, rise and fall of water level