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The lower semi-open box (LSOB) girder is widely used in large-span bridges due to its favorable stress performance and cost-effectiveness. The vortex-induced vibration (VIV) of the LSOB girder was studied by using a 1:55 section model wind tunnel test. The test results show that there are obvious vertical and torsional VIV intervals with significant amplitudes at each angle of attack with the damping ratios of about 0.35%. The CFD numerical simulations indicate that VIVs of LSOB girders are mainly induced by two types of vortices: one at the inclined webs of leeward wind nozzles and another at the upstream upper surfaces of the deck. Conventional aerodynamic vibration suppression measures, such as the lower central stabilizing plate and the guide vane at the gantry rail, demonstrate that the lower central stabilizing plate can reduce the VIV response at all wind angles of attack, decreasing vertical amplitude by 40%–50% and torsional amplitude by 10%–25%. Although adding a guide vane further optimizes the VIV performance, its effect is limited. Installing the lower central stabilizing plate helps redistribute vortices below the girder, transforming large vortices into smaller ones, most of which do not detach, thereby reducing vortex shedding intensity in the wake area. The bilateral guide vanes at the maintenance car track further weaken the wake vortices at the diagonal webs on the leeward side.