To study the axial compression behavior of concrete-filled steel tube (CFST) at early-age, axial compressive tests of CFST stub columns at different concrete ages were carried out. The bearing capacity and load-strain curve at different ages were obtained. The failure modes, load-strain curves, and the development rules of bearing capacity with age were analyzed. Based on the existing stress-strain relationship of early-age concrete, the constitutive relationship of mature concrete in the GB 50010 was extended to include the case of concrete at early-age. Combined with the above mentioned tests, the finite element model for early-age CFST was established and the axial compression behavior of circular and square CFST at early-age was analyzed. The results show that the confinement effect increases with the growth of age and the confinement effect and load properties of the circular section aresignificantly higher than those of the square section. Based on the bearing capacity of CFST in the mature period, the strength capacity of CFST stub columns at the early age was established by introducing the coefficient of confinement effect due to concrete age. Uniform design method was used to simulate the age-bearing capacity of circular and square CFST stub columns to verify the applicability and high accuracy of the formula. This study provides references for the safe construction of CFST.