Abstract: The vibroflotation pile method is used to replace and compact the soft foundation with permeable materials such as gravel to form composite foundation, and in the past, the attention to shear strength of this kind composite foundation is little. Due to the compaction and replacement of gravel and soil around piles, difference is shown between gravel materials and actual vibroflotation gravel piles, regardless of particle composition or stress-strain characteristics. Based on field detection, sampling for vibroflotation gravel piles and gravel materials and indoor geotechnical tests to analyze the difference of physical and mechanical properties between vibroflotation gravel piles and gravel materials, the researches show: (1) The content of fine particles in actual piles is significantly higher than that of gravel materials, which mainly comes from the mixing (squeezing) of soft soil around piles during vibroflotation process; (2) The maximum dry density of piles is higher than that of gravel materials, which is directly related to the poor grading of gravel materials and mixing of fine particles; (3) Refer to actual compaction degree in the field, large-scale triaxial tests are carried out on gravel materials and two kinds of piles with different pile spacings, the strength of piles is about 10% lower than that of gravel materials, considering the difference in stress-strain characteristics, if the strength parameter is taken according to 5% axial strain, the strength of piles is more than 20% lower than that of gravel materials, and mixing of soil around piles has a significant deterioration effect on the shear strength and also the stress-strain characteristics of the piles. Based on the above initial conclusions, the modified suggestions for strength design parameters of composite foundation are proposed.