Vortex shedding and drift are key characteristics around the bridge girders during VIVs，and therefore it is necessary to reveal VIVs mechanisms of bridge girders from the perspective of vortex dynamics. A simplified vortex model was constructed from the perspective of aerodynamic work. Taking a typical streamlined-closed box girder as an example，a simplified wortex model was constructed from the perspective of aerodynamic work. Combined with the aerodynamic time-frequency characteristics of the bridge girder from wind tunnel experiments and the flow field characteristics around the girder based on numerical simulation method，the above model was verified and then the multi-order VIVs lock-in range mechanisms of the girder were revealed. The results indicate that the Strouhal number of the separation vortex characterizes energy effects of the vortex aerodynamics，which can be expressed as a positive integer multiple of the ratio of vortex-drift velocity to oncoming flow velocity，implying that a separation point can excite multiple VIVs lock-in ranges. There were 3 order lock-in ranges of vertical VIVs for the girder. Both the 2nd and 3rd lock-in ranges are excited and sustained by the large-scale separated vortexes separating at the leading edge and periodic drift in the drift distance between the separation point and the trailing edge. Especially，it takes about 2 and 1 vibration cycle for the separation vortexes to traverse the drift distance in the 2nd and 3rd order VIVs lock-in ranges，respectively. Therefore，they are dominated by the 2nd and 1st order simplified-vortex modes originating from the leading edge，respectively. This study verifies the rationality of the simplified-vortex model to deduce the vortices evolutionary characteristics around the bridge girder and provides a new methodology for VIVs mechanism of the bridge girders.