Abstract: The pultruded glass fiber reinforced polymer (GFRP) tube-self-compacting concrete (SCC)-steel tube composite column exhibits superior mechanical properties, lightweight and high strength, excellent corrosion resistance and convenient construction, making them highly suitable for applications in bridges, high-rise buildings. Axial compression tests were carried out on five composite medium-long column specimens, combined with load-displacement curve and load-strain curve, considering three important parameters: SCC strength grade, GFRP tube thickness and CFRP cloth reinforcement length at the end. The ultimate bearing capacity, constraint effect coefficient, ductility coefficient, stiffness degradation rate and other performance indexes were systematically studied. A load-bearing capacity calculation model for composite medium-long columns was established using the twin shear unified strength theory, incorporating a stability coefficient. The results indicate that increasing the SCC strength grade can effectively enhance the bearing capacity, but the smaller the confinement effect coefficient, the more significant the stiffness degradation; the ultimate bearing capacity and constraint effect of the specimen with large thickness of GFRP tube are greater, but the stiffness degradation rate is close to that of the specimen with large thickness of GFRP tube; extending the length of the CFRP cloth reinforcement at the ends mitigates stiffness degradation, but it has limited effects on the ultimate bearing capacity and confinement effect. The bearing capacity model of the composite medium-long columns, adjusted with the stability coefficient, has good calculation accuracy and stability, with an optimal model bearing capacity coefficient mean value of 0.992 and a standard deviation of 0.047, providing a reliable basis for the future design of composite medium-long columns.