The tests of 9 composite concrete-filled square thin-walled steel tubular (CCFSTST) columns with longitudinal stiffeners under eccentric compression were carried out. The test results indicated that the load-carrying capacity decreased with the increase of eccentricity and slenderness ratio, but increased with the increase of core concrete strength. Based on the finite element model verified by the test results, the working mechanism of CCFSTST columns was studied and the parametric analysis was conducted. It can be found that the confinement of the outer steel tube is focused on the corners of the cross-section, the majority of the load is resisted by the concrete and the ductility of the composite column is enhanced by filling core concrete. With the increase of the steel yield strength, steel ratio of inner concrete-filled steel tube and diameter to width ratio, the abscissa and ordinate values of the equilibrium point in the relative axial compression versus relative moment relationship decreased, but they increased with the increase of concrete strength. Meanwhile, with the increase of slenderness ratio, the axial compression versus moment relationship trended to be a straight line. Finally, the simplified model was proposed to predict the load-carrying capacity of CCFSTST columns under the eccentric compression.