In this study, sectional model wind-tunnel tests are conducted to investigate the vortex-induced vibration (VIV) of semi-closed girders with different aspect ratios. The influencing factors, including the wind attack angle and Scruton number Sc, are considered. The VIV is compared among semi-closed girder sections with three aspect ratios and unclosed widths. Numerical simulation is performed to investigate instantaneous vorticities and mean streamlines around the girder sections. The findings indicate that, at a wind attack angle of +3°, vertical VIVs are consistently observed in the girder sections with three different aspect ratios. Moreover, a semi-closed section with a large unclosed position exhibits a vertical VIV at a wind attack angle of 0°. As Sc increases progressively, the discrepancy in the vertical VIV amplitude of the main girder diminishes gradually. At the same Sc, the vertical VIV amplitude of a girder with a smaller aspect ratio is larger than that of a girder with a larger aspect ratio. A large width of the unclosed position can deteriorate the VIV, thus affecting the flow structure surrounding the girder section. At a wind attack angle of 0°, for a girder section with a large aspect ratio and small unclosed width, the lower vortices propagate along the girder section. Additionally, the vortices propagate away from the bottom plate of the girder section when the girder section features a large unclosed width at a wind attack angle of 0°. At a wind attack angle of +3°, the vortices at the unclosed position propagate along the section toward the leeward side. A positive wind attack angle intensifies the flow separation, and the size of the vortices at the unclosed position increases to a certain extent. These findings can provide a reference for the wind-resistant design of semi-enclosed main girders.