Abstract: Establishing a stability analysis approach for CFG pile composite foundation requires a thorough understanding of its instability and failure characteristics in subgrade engineering. For the deep soft clay foundation under embankment, three groups of centrifugal model tests are conducted in this study. The self-developed in-flight sand release device is used to simulate the staged embankment construction. In order to monitor the failure sequence of the piles, the model piles are made by wrapping graphite sticks with mortar. The photogrammetry is used to obtain the contours of foundation displacement. Finally, the bearing and deformation characteristics of the composite foundation are analyzed along with their sliding surface. The tests show that as the embankment height increases, the model piles near the embankment toe firstly fail, followed by the failure of piles beneath the shoulder and center of the embankment, exhibiting a progressive failure feature. After the global instability is triggered, the sliding surface of the composite foundation is nearly a circular slip plane. The failure modes of piles are closely linked to the deformation of the surrounding soil. Tensile bending failure is observed for the piles near the slope toe, where the foundation soil heaves, but the compressive bending failure of piles occurs near the embankment shoulder, where the embankment load is relatively large. The pile cap has a significant effect on the load distribution between the piles and the soils. As the axial force of the piles increases, the flexural tensile capacity of the piles is enhanced, which benefits the stability of composite foundations.