Abstract: The behavior of UHPC confined concrete (UCC) composite columns can be improved by the synergistic operation of UHPC and restraint stirrups due to the "bridge" effect of steel fibers embedded in the outer UHPC. In order to quantify the confining effect of the outer UHPC and the stirrups on the inner concrete, and to develop a calculation method for axial compression capacity, an analysis was conducted on the test data of 49 UCC composite columns. Furthermore, seven UCC composite columns, which exhibit various degrees of confining effects, were subjected to compression tests. These tests considered variables such as the interface position of the stirrups, the wall thickness of UHPC and the interface roughness. The failure mechanism due to axial compression was revealed. Utilizing the principles of circumferential deformation and the theory of thick-walled cylinders, an analysis was conducted on the internal force and deformation characteristics of the inner concrete and the outer UHPC. The binding force exerted by the outer UHPC on the inner concrete was computed based on the circumferential strain coordination and the force balance equation. A novel method for calculating the vertical compressive stress of concrete in a confined state and UHPC in a tension state was proposed. The formula for calculating the axial compressive capacity of UCC composite columns was derived based on the superposition principle. It was then applied to computing the axial compressive capacity of 56 UCC composite columns, indicating that the calculation formula for the compressive capacity of UCC composite columns exhibits a high degree of accuracy.