Nonlinear constitutive law was used in this paper. On the basis of plain section assumption, through isoparametric element theory, nonlinear secant and tangent stiffness matrices of section were derived. Nonlinear equation was solved by Newton-Raphson method. The bearing mechanism under the eccentric compression was revealed by changing eccentricity and position coefficient. The result is found to be in good agreement with the experimental data, and it shows that large and minor eccentricity failure mode, which are similar to reinforced concrete, are existed in steel tube-reinforced concrete column. In addition, force-moment correlation curve is derived. By calculation and comparison. The ultimate capacity is found to be greatly underestimated in the current technical specification.