A typical streamlined closed-box girder is taken as the research object in this paper. Utilizing the wind tunnel tests with a section model, vortex-induced vibration (VIV) responses of the grinder section were obtained, and the contribution values of the distributed aerodynamic torques were analyzed for both the original and improved girder designs (the improved girder with spoilers and the improved girder with guide vanes for maintenance rails) under typical wind conditions. By combining the simplified vortex method (SVM) with numerical simulation, the torsional VIV of the bridge girder and its suppression mechanism with additional aerodynamic countermeasures were further revealed. This paper provides a new methodology for analyzing the VIV mechanism of bridge girders and the VIV suppression mechanism of aerodynamic countermeasures. The results reveal that an obvious torsional VIV phenomenon was observed on the original girder, with a maximum amplitude of 0.112°. After adding guide vanes for the maintenance rail, the torsional VIV amplitude of the section was reduced by 35.7%, and the torsional VIV phenomenon disappears after the addition of the spoilers on the sidewalks’ handrails. For both the original and guide vane girders, the contribution values of the distributed aerodynamic torques on the upper surface to the global vortex excited force (VEF) were much greater than those on the lower surface. The VIVs of the original and guide vane girders were dominated by the periodic drift of the large-scale vortices generated from the leading edge to the trailing edge on the upper surface. The drift time of the vortices was approximately 2.5 vibration cycles, which corresponds to the second-order torsional simplified vortex mode. After the installation of guide vanes for the maintenance rails, the contribution values of the distributed aerodynamic forces to the global VEF were significantly reduced, and the scale and intensity of vortices around the girder were reduced, so the VIV amplitude decreased. After adding spoilers, the contribution values of the distributed aerodynamic forces to the global VEF became more evenly distributed and were greatly reduced.Spoilers inhibited the formation of separation vortices at the leading edge of the upper surface, effectively eliminating the VIV phenomena.