Taking the foundation treatment of gravel pile in a proposed hydropower station as the background， an indoor experimental sample preparation method of soft clay composite samples with gravel core was developed， which can improve the consolidation efficiency of remolded soft clay and reduce the disturbance of the samples. The large triaxial tests of composite samples with different confining pressures and gravel core replacement rates were carried out. The results show that composite specimens exhibit strain hardening characteristics under small confining pressures， while they generally exhibit softening characteristics under high confining pressures， and the softening characteristics are more pronounced with a smaller gravel replacement rate. At the same confining pressure， the initial deformation modulus of the composite specimen increases with the increase of the sample area replacement rate. At high confining pressure and low replacement rate， the shear failure surface of the soft clay gravel core composite sample is obvious. At low confining pressure and high replacement rate， there is a more obvious bulging phenomenon in the middle of the composite sample. The maximum bulge amount of the gravel core generally increases with the increase of confining pressure and replacement rate. The current code method overestimates the internal friction angle of the composite specimen （foundation）， and the larger the area replacement rate， the greater the difference between the calculated values and the experimental results. Under high confining pressures， the traditional stress superposition method overestimates the bearing capacity of the soft clay vibroflotation replacement gravel pile composite foundation and underestimates the settlement of the composite foundation. The higher the replacement rate， the greater the deviation stress error between the stress superposition method and the large triaxial test of the composite sample.