The force method is a classical approach for bending analysis of statically indeterminate structures. Based on the similarity between the bending and restrained torsion problems of a continuous box girder， the force method is applied to the analysis of restrained torsion， addressing the critical limitation of the graph multiplication method， which fails to calculate flexibility in such cases. Initial parameter solutions to the differential equations governing the restrained torsion of thin-walled box girder are used to derive theoretical expressions for state vectors （twist angle， warping displacement， bimoment and torque） in the released structure subjected to unit redundant and loads （concentrated torque， uniformly distributed torque， concentrated bimoment and uniformly distributed bimoment）. The theoretical solutions are then applied to calculate the flexibility of the released structure. The superposition method is implemented to conduct the post-processing of state vectors. Theoretical formulae and post-processing method of state vectors lay a concrete groundwork for the force method applied to the restrained torsion analysis of continuous thin-walled box girder. The example results by the force method are consistent with the exact finite elements in a continuous thin-walled box girder， which demonstrates the precision of the force method applied to the restrained torsion analysis of a continuous thin-walled box girder. The integral force method presented in this paper is based on the analytical solutions of state vectors in the released structure and the superposition method for the post-processing of state vectors. Critical questions are solved to expand the applications of the force method.