Abstract: The mechanical characteristics of pre-stressed concrete beams under combined forces of bending, shear and torsion are complicated. Accurate calculations for the purpose of the structural design and functional analysis require precise knowledge of the torsional strength of each concrete structural member. This paper proposes a formula to calculate the torsional strength of structural members under conditions of small torsion-bending ratio, using the plate-truss model of pre-stressed concrete beams under combined forces of bending, shear and torsion. The calculations focus on the case of the pre-stressed concrete box-section beam. These calculations are based on the equilibrium conditions of forces and moment, and the deformation and constitutive relationships, as well as the failure criteria of the materials. Results agree well with experimental data. Therefore, the plate-truss model theory of pre-stressed concrete beams under combined forces is verifyied.