A circular carbon steel tube was added to the core concrete inside square stainless-steel tubes, the constraint effect coefficient to the core concrete and the ductility of the composite CFST sections could be improved remarkably, as a result, the anti-corrosion protection and fire protection of traditional steel structures could be reduced, furthermore, the disadvantageous influence of the demolished concrete lumps (DCLs) in new concrete could also be improved. Sixteen composite-sectioned concrete-filled stainless steel tubular (CS–CFSST) stub columns filled with high-strength demolished concrete lumps (DCLs) were tested. The influence of factors such as the replacement ratio of the demolished concrete lumps, confinement factor, and different concrete strength of new or demolished concrete on the mechanical performance was investigated. According to the testing results, the axial bearing capacity of the CS–CFSST stub columns decreased with the increase of the mix ratio of DCLs, the maximum decreasing amplitude of the bearing capacity was 6.8% with the replacement ratio of 20%. The failure modes of the CS–CFSST stub columns were quite familiar with those of normal CS–CFSST stub columns filled with normal fresh concrete. Two types of failure modes, drum-shaped and shear failure modes were found in the inner circular CFST stub columns, and excellent ductility and residual bearing capacity were also found as the load-deformation curves kept horizontally or ascended slowly, the residual bearing capacity of all the specimens were higher than 63% of its bearing capacity. The simplified method of constraint effect coefficient in the CS–CFSST stub column was proposed, and four calculation methods of the bearing capacity of the CS–CFSST stub columns infilling with DCLs were compared, coincident results were obtained using the calculation method considering the ultimate bearing capacity reduction of the circular CFST section