Abstract: The jet grouting jacked composite pile is a kind of new patented technology which is suitable to the construction in the existing buildings with low clearance. The vertical bearing mechanism of the jet grouting jacked composite piles is studied through the indoor full size tests and compared with that of the jacked piles, jet grouting piles and cast-in-place piles. The tests show that the jet grouting jacked composite pile is mainly characterized by a kind of friction pile. The vertical bearing capacity of the jet grouting jacked composite pile is higher than that of the jet grouting pile and the cast-in-place pile with the same diameter. The vertical bearing capacity of the jacked pile is the lowest due to the smaller diameter. The jet grouting pile fails due to the fracture of the pile shaft materials on the top of the pile under the ultimate load, and the side resistance and tip resistance do not fully play their roles; since the pile body strength of the jet grouting jacked composite pile is improved by jacking the core pile in the jet grouting pile, the side resistance can fully play its roles and improve the bearing capacity. Compared to that of the cast-in-place pile, the interface between jet grouting jacked composite pile and soil is irregular and the shear behavior of the interface is improved by grouting, and the vertical bearing capacity of the jet grouting composite pile is higher than that of the cast-in-place pile. Based on the test results and from an engineering practical point of view, a modified expression for the vertical ultimate bearing capacity of single jet grouting jacked composite pile is obtained by introducing an amplification coefficient for the side resistance and a reduction coefficient for the tip resistance to hat of single cast-in-place pile. The values of the relevant parameters are recommended.