To improve the accuracy of the classic Vlasov torsional theory when it is applied to analyze deep beams with open cross sections, restrained torsional calculation formula was derived based on the effect of shear deformation and the Vlasov torsional theory. Equilibrium differential equations were obtained in consideration of the influence of shear deformation, and the solutions were obtained through initial parameter method. Then, U-shaped RC beams (span of 6.65 m and 3.325 m) were calculated by the proposed calculation method, Vlasov torsional theory, and ABAQUS simulations. It was found that results of the two calculation methods were close to each other for the long U-shaped beam with large span-height ratio (when l/h＞10, the variation was within 20%), which was consistent with the experimental and simulation results. However, for the short U-shaped beam with small span-height ratio, the torsional rotation was significantly underestimated by the Vlasov torsional theory (when l/h＜6, the variation exceeded 40%), while the results obtained from the proposed method well coincided with the experimental and simulation results. Therefore, the method proposed in this paper could overcome the limitation of ignoring shear deformation in the classic Vlasov torsional theory, which is suitable for the calculation of torsional deformation of open thin-walled members with both large and small span-height ratios.