Experiments are performed on three spring-supported separated-box girder section models in a closed wind tunnel to investigate the vortex-induced vibration (VTV) characteristics. A particle image velocimetry (Ply) instrument is employed to obtain the flow field around the models. Four VIV-sensitive velocity intervals are observed for the test model, in which vertical and torsional oscillations are repeatedly induced. Comparisons with the control groups show that vortex shedding in the gap is the crucial factor that induces VIVs. Wake-induced vibration (WIV) is observed to occur with VIV in the gap, accompanied by a dramatic magnification in vibrational amplitude. The WIV becomes weaker and disappears as the wind velocity increases, and the wake vortex-shedding mode changes from the classic 2S mode to a 2(2S) mode. The vortex-shedding frequency shows that it is a special lock-in mode of 1:2 phase-locking, which is found by taking the vibrating model and vortex-shedding as two coupled oscillators.