Abstract: To understand the mechanical behavior of composite structures with high strength materials, seven ultra-high strength concrete (UHSC) filled high strength steel tube (C_uFS_hT) columns under axial load were tested. The failure modes, load-axial displacement curves, load-deflection curves, and steel strains of the specimens with different length to diameter (L/D) ratios were investigated. The results show that the influence mechanism of the L/D ratio on the overall buckling of C_uFS_hT columns is similar to that on ordinary CFST columns. However, the C_uFS_hT columns exhibited sudden bending behavior earlier due to the significant elastic behavior of its composite materials. A finite element model verified by the test results was used to explore the influence of section parameters on the stability curves. The results indicate that the influence is negligible if the slenderness ratio is regularized by the column strength. The column strength is influenced by the difference in the confinement action of different material combinations. Taking the regularized slenderness ratio as an independent variable, a new calculation method of the stability coefficient with an extensive application scope was proposed. It has good accuracy in predicting the strengths of C_uFS_hT columns under axial load.