The simplified “fish-bone” model is commonly employed for the analysis of wind-induced responses of truss-stiffened suspension bridges, and one key step in establishing such a model is the calculation of the mass moment of inertia for the stiffening truss. Due to numerous members in a stiffening truss section, the calculation process of the moment of inertia is usually complicated and imprecise. Therefore, a new method is proposed based on the change in torsional frequencies of a cantilever truss girder caused by attaching the additional mass moment of inertia on the truss nodes, where the theoretical background is given, and the applicable conditions are also investigated. By taking a cantilever truss girder as an example, numerical simulation results show that the proposed method provides the best accuracy when the known mass moment of inertia is uniformly applied to all section nodes or to the nodes of chord joints, and the slenderness ratio of truss girder exceeds 20. Finally, the mass moments of inertia of stiffening truss are obtained by the proposed method, and are then used to establish the simplified “fish-bone”models for several selected suspension bridges. The torsional frequencies obtained by the simplified “fish-bone” models agree well with those predicted by their detailed models, which validates the effectiveness of this simplified method.