Abstract: On the basis of classical theory of free torsion and the deformation characteristics of a truss, the torsional deformation is simplified to study the torsional behaviors of isosceles triangular space trusses with symmetrically-distributed web members. The methods of calculating torsional center, elastic torsional stiffness, and yield torque of a truss are proposed, and the mechanism of truss failure is analyzed. In order to verify the calculation method of the torsional behaviors of a truss, the torsional test of a truss is done, and the error between the value of calculating elastic torsional stiffness and the test value of the test member is less than 10%, the same as a yield torque. The Finite Element Method (FEM) is employed to simulate and verify the test member and calculation method. The results show that the calculation values, test values and FEM values are well consistent with each other. A practical calculation method is proposed for the free torsion of isosceles triangular space trusses.