Abstract: The calculation method of the torsional resistance of reinforced concrete (RC) box girders is based on the variable angle space truss model. The angle of concrete compression diagonals is the critical parameter in the model, which has a significant influence on the calculation results of the torsional resistance. However, the calculation methods of the angle of compression diagonals are different in various research and national codes. To investigate the actual principal direction of the slab in an RC box girder under pure torsion and compare it with the angles of compression diagonals calculated using different code methods, a nonlinear finite element analysis program for RC box girders was established based on the nonlinear layered shell elements and the modified compression field constitutive models. The program was validated by a series of RC panel tests under pure shear, which showed that the numerical program accurately simulated the whole-process deformation developments and the failure modes of the panels. Based on the validated numerical results, the whole-process variations and the ultimate values of the actual truss angle of compression diagonals, plastic truss angle of compression diagonals, principal strain angle of elements, principal stress angle of concrete elements and principal stress angle of RC elements of the panels were compared and analyzed. The analysis results indicated that the principal strain angle (representing the actual principal direction of the panel) gradually rotated towards the stiffer direction after the cracking and began to rotate at a faster speed when the applied load approached the yield load. The actual truss angle of compression diagonals had the largest difference with the principal strain angle at the beginning and approached the principal strain angle with increasing load. At ultimate state, the actual truss angle of compression diagonals and the principal stress angle of concrete elements agreed well with the principal strain angle, while the plastic truss angle of compression diagonals was significantly smaller than the principal strain angles. Finally, the torsional resistances calculated based on different ultimate angles of compression diagonals, were evaluated. The calculation method in Chinese code, which uses the plastic truss angle of compression diagonals, is on the unsafe side, and the degree of unsafety depends on the reinforcement strength ratio.