Abstract: Deep mixing(DM)method has become one of the most commonly-used techniques to improve soft soils under embankments, which can be used to reduce settlement, increase bearing capacity and enhance stability of foundation. However, the assessment of factor of safety against slope stability of DM column-supported embankment over soft soil is still a concern in practice, owing to a lack of well understanding of its failure modes. The existing method only considers the contribution of shear strength of DM column against the slope failure, which would overestimate the factor of safety of embankment stability. Accordingly, this paper presents a model test to investigate the failure modes of slope stability of deep mixed column-supported embankment. 3-D numerical model corresponding to the model test was simulated to investigate the evolution of plastic zone in the process of slope failure. The results indicated that the DM columns under different locations under the embankment had different failure modes. In the process of slope instability, the DM columns located under the embankment crest were mainly subjected to the vertical load, firstly resulting in a compression and shear failure. After that the horizontal earth thrust carried by the columns under the slope of embankment increased, resulting in these DM columns having a bending failure.