To investigate how end restraints influence restrained torsion in a double-layer steel truss girder, the girder was equivalently modeled as a steel box girder using the orthotropic steel deck and web-shear equivalence principles. Based on an energy variational formulation, the governing differential equations and boundary conditions for restrained torsion were derived, and the effects of different end constraints were quantified. Numerical studies indicate that the analytical solution agrees well with finite-element results for the equivalent box-girder model. For the fixed-hinged (left-fixed/right-hinged) case, the longitudinal distribution of warping normal stress is steeper, and the warping stress near midspan is about twice that of the simply supported and fixed-fixed cases. Secondary shear flow significantly reshapes the total shear-flow pattern: the maximum total shear flow occurs at the midpoint of the bottom plate and is approximately two times the free shear flow. Compared with a simply supported girder, the fixed-hinged girder shows a 1.5-fold increase in secondary torque at midspan. Under the most unfavorable vehicle loading, the warping bimoment generally decreases along the span, and stronger end restraints produce steeper distributions of both warping bimoment and secondary torque, leading to more pronounced end-region effects and higher demand near restrained boundaries.