Abstract: In order to study the torque-strain relationship of concrete filled circular steel tube (CFST) under pure torsion, the whole process of CFST under pure torsion is divided into three stages: before concrete cracking, before steel tube yielding and after steel tube yielding. On the basis of this division, considering the stress state and interaction between steel tube and concrete, the shear strengths of steel and concrete are derived from the Unified Strength Theory. Taking into account the relationship between shear strain and normal strain, adopting the ideal elastic-plastic constitutive relation of steel, and accounting for the softening effect of concrete, the piecewise function model of torque-strain and the calculation method of torsional bearing capacity of CFST under pure torsion are derived based on limit equilibrium theory. The theoretical model and torsional bearing capacity are in good agreement with the test results. On this basis, the influences of steel pipe diameter, steel strength, concrete strength and steel content ratio on the torque-strain relationship are analyzed, and some application suggestions are put forward. The theoretical model in this paper describes quantitatively the interaction between steel tube and concrete, which can be applied to the analysis and design of CFST under pure torsion.