The behavior of thin-walled round-ended concrete-filled steel tubular (RECFST) short columns loaded axially is characterized by the local-buckling of the flat steel walls and the concrete confinement exerted by the round-ended steel walls. A numerical modeling method implementing fiber analysis is presented in this paper for the determination of the behavior of short thin-walled RECFST columns subjected to axial loads. The mathematical simulation method explicitly incorporates the progressive local buckling of the flat steel walls as well as concrete confinement induced by the round-ended tube walls. A new constitutive model is developed by analyzing the available test results for determining the lateral pressures on the concrete confined by the round-ended tube walls. A coefficient of strength degradation is provided to quantify the residual strength of the encased concrete. The independent test results are utilized to assess the accuracy of the computational modeling scheme. Comparative investigations are carried out to evaluate the accuracy of the numerical models of RECFST columns presented by other investigators. The computer modeling program designed is utilized to quantify the structural responses of thin-walled RECFST composite columns subjected to the variations of material and geometric parameters. A design equation is given for designing thin-walled RECFST columns considering local buckling and concrete confinement effects. The applicability of the current design standards for circular and rectangular CFST columns to RECFST columns is assessed by utilizing experimental data. It is found that the current design codes need to be modified to be used to design RECFST thin-walled columns. The proposed computer simulation program and design equation are demonstrated to accurately yield the performance of RECFST columns under axial loading.