To accurately predict the shear capacity of square concrete filled steel tube (SCFST) columns, a theoretic model was established based on modified compression field theory. In this model, compatibility and equilibrium relationships were taken to consider the axial-flexure-shear interaction, and a general formula of shear capacity of SCFST was deduced by dissecting shear mechanism. Due to two different failure modes observed in previous tests, two different failure criterions were given respectively. The proposed model was applied to the 17 existing column specimens, which predicted well the shear capacities of the test specimens. The results show that the proposed model not only can be utilized to evaluate shear capacity of SCFST subjected to axial force, flexural moment, and shear force, but also can be employed to verdict different failure modes. The calculation results of the proposed model are credible, and the model is effective.