Stiffened deep mixed columns （SDMCs） can effectively increase bearing capacity， reduce settlement and enhance stability when they are used to improve soft soil. Currently， the researches on SDMCs have been mainly focused on the engineering properties of SDMC under vertical loading， leaving a gap in improving understanding of failure mode of SDMC under embankment load as well as its calculation approach. This paper conducted a series of 1g gravity scale-reduced model tests， including embankments supported by natural soft soil， deep mixed （DM） column-reinforced soil， and SDMC-reinforced soil. The instability of embankments supported by different foundations were analyzed through a comparison of deformation of embankment， stresses on column head and soil， strains in column， and failure modes of column. The applicability of the existing stability calculation methods were assessed. The results showed that the SDCM columns effectively restrained the soil deformation， namely， its settlement on the embankment crest and the ground heaving outside the embankment were approximately 61% to 71%， and 12% to 46% of those in the model test with DM columns， leading to a significantly enhancement on the global stability of the embankment. The SDCM columns failed progressively， and it may have failure modes of compression， a combination of compression and bending， and bending， depending on the column positions under embankment and column stiffness. The slip plane did not exactly go through the broken positions of column. The equivalent shear strength method had applicability to the stability analysis of SDMC-supported embankment as compared with other methods considered in this study.