In view of the deviation between the design and calculation results of underwater compacted sand pile composite foundation and the engineering practice，the mechanical characteristics are analyzed and studied. The test of soft soil foundation reinforced by compaction sand pile in the Yangshan deepwater port area is simulated and analyzed. Research conclusions of settlement characteristics，bearing capacity，transmission and variation law of load along the depth of the composite foundation as well as the load-sharing effect of sandpile and soil between piles are obtained. Several different related parameter settings of the composite foundation model are calculated and compared，and the influence of the changes of parameters on deformation and force characteristics of the composite foundation is analyzed. This study provides a reference for the design of underwater soft soil foundation reinforcement of offshore deep-water ports.