The engineered excavation in mountainous areas for land reclamation is creating numerous single-staged high embankment slopes (height 40 ∼ 50 m) on the hilly loess areas in China. With an increasing amount of failures of loess slopes to be found in major cities, embankment slopes can pose great risk to the local community and cause damages to the properties in the surrounding areas. In this article a case study disclosed a complete failure process of a high fill slope (max. height > 40 m) although it had strengthened by a combining technique of reinforced retaining walls with anchor piles. The deformation of the retaining structures ensued from the loading imposed by the construction, with cracks formed at the bottom of the pile board and retrogressed upwards. The proportion of the cracks with opening exceeding 3 mm increased with the progress of the backfill. The deformation process of the hillslope was recorded by the IBIS-L terrain deformation monitoring system at resolution of 0.1 mm. The deformation process can be divided into three phases, i.e. creep—slip—collapse, to respond to the deformation development at different stages. The instability criterion is proposed by analyzing the results of deformation characteristics of the hill slope. It indicates that the slope failure will occurs in case the slip surface be formed at s tangent angle > 75° on the deformation curve with rapid rise of acceleration from 0 mm/h2 to 8.9 mm/h2.