Abstract: The jet grouting jacked composite pile is a kind of new patented technology which is suitable to construct under low internal headroom conditions within existing buildings. The vertical bearing mechanism of jet grouting jacked composite piles was studied by indoor full size tests, and contrasted with that of jacked piles, chemical churning piles and cast-in-place piles. The tests show that: the jet grouting jacked composite pile is mainly a kind of friction pile; the vertical bearing capacity of the jet grouting pile is higher than that of the chemical churning 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 less diameter; the failure mechanism of the chemical churning pile is strength fracture of pile shaft material on the top of the pile under ultimate load, and the side resistance and tip resistance doesn’t play its roles sufficiently, the pile body strength of the jet grouting jacked composite pile is make up by jacked the core pile in the chemical churning pile, so the side resistance and tip resistance can play its roles sufficiently, therefore the bearing capacity is higher; compared with the cast-in-place pile, the shaft of the jet grouting jacked composite pile is wedge-shaped and the interface is irregular between the pile and soil, the interface shear behavior is improved by grouting, the vertical bearing capacity of the jet grouting composite pile is higher than that of the cast-in-place pile. According to the test results and from an engineering practical point of view, the expression of the vertical ultimate bearing capacity of single jet grouting jacked composite pile is given, which is amended that of single cast-in-place pile by introducing amplification coefficient for side resistance and drawdown coefficient for tip resistance, and the value of relevant parameters is suggested.