Abstract: In recent years, the constructions of airport, highway and railway transportation network and urbanization in the loess hilly areas have been carried out more and more frequent. They have formed a large quantities of high loess embankment slope. Although the annual rainfall of these areas is semiarid, rainfall is the main factor for the failure of loess slopes. This paper analyses the in-situ seepage experiment and the transient unsaturated seepage under the conditions considering the fractures and the mechanical properties of saturated loess. It examines the failure mechanism of high fill retaining loess slope. The results show the following. (1) The maximum infiltration depth of the test pit center is 1.30m after 3 days in the in situ seepage experiment. The maximum depth of the wetting front is 0.20m below the maximum infiltration depth. (2) When the confining pressure is less than 300kPa, the axial strain of the saturated soil increases to about 20%, and reaches the steady state. Under the condition that static driving shearing stress is larger than the steady shear strength, the local flowslide failure of the high fill slope can occur. (3) In general, the formation mechanism of deformation and failure of the retaining high embankment slopes is as follows:push creeping-retaining structural failure-progressive slip shear-traction collapse.