The mechanics performance of reinforced concrete members subjected to axial compression, bending, shear and torque is very complex. Several calculation models have been established to estimate the torsional capacity of combined torsion members; however, the calculation results of different models have a great variation. In this paper, variable-angle space truss model is adopted to analyze the mechanics performance of reinforced concrete members subjected to combined torsion. With respect to various shapes of specimens, various load modes, and various strength of concrete, a unity equation about torsional capacity of combined torsional member is obtained. Based on the unity equation, the torsional capacity of 59 combined torsional specimens is calculated. In contrast with the equation of ACI and the code of China, the torsional capacity calculated by the unity equation agrees well with the results of experiment and much better than the results of ACI code and China code. It is concluded that the unity equation can provide valuable reference for calculation and design of combined torsion members.