To investigate the failure mechanisms of the geogrid-reinforced and pile-supported foundation in which the embedded piles and floating piles exist at the same time, the centrifuge model test with the similar constant of 1∶100 was performed based on a typical project of a high speed railway located in the coastal areas of Zhejiang Province. The deformation characteristics and failure modes of such foundation on soft soil with overlying sloping base were analyzed in detail. According to the centrifuge model test, the conclusions are drawn as follows: most vertical and horizontal displacement occurred during the construction phase and the standing time, and increased continuously in operating phase. Meanwhile, the differential settlement increased gradually with the operation time. The distribution of the composite foundation settlement was asymmetric, and a settlement basin appeared after the test. Under the upper load, the settlement of the composite foundation was mainly caused by the underlayer compaction, the upward and downward pricking destruction of the pile, and tilting or transversal displacement of the pile. The horizontal displacement of the pile head increased gradually along the downward direction of the overlying sloping base, and the soil flowed around the piles in varying degrees. The pile head vertical displacement distributed unevenly and presented an asymmetric shape of "V". The embedded piles suffered bending tensile failure or bending-shear failure; piles on the overlying sloping base suffered bending-shear failure or tilting failure; and floating piles suffered tilting failure and transversal displacement failure. Test results indicate that it is unreasonable that this kind of foundation is reinforced by equilong piles, and the effective reinforcement measures for this kind of high speed railway foundations still need further research.