Abstract: Because of its excellent limit-span and flutter stability, the separated triple-box girder is adopted by the system of long-span bridges more and more widely. In view of the fact that the triple-box girder is extremely sensitive to the wind loading and prone to vortex-induced vibrations, the study took the separated triple-box girder of one typical long-span bridge under construction in China as the research object. Through vibration tests, wind speed tests and smoke-wire flow visualizations in the wind tunnel test, the vortex-induced vibration (VIV) characteristics and flow field characteristics of the bare separated triple-box girder are researched. To reveal the VIV mechanism of the separated triple-box girder, the upstream gap, the downstream gap and both gaps are covered, respectively. The VIV responses and the flow field characteristics of the girder model are compared at various VIV stages at the wind attack angle of 0°. The results showed that: the VIV of the separated triple-box girder had five vibration stages (Beginning, Ascent-1, Ascent-2, Peak, Descent), accompanied with two main types of vortices (Karman vortex, Motion-induced vortex), and the Karman vortex in the upstream gap induced the occurence of VIV and the motion-induced vortex at various locations led to the deterioration of the VIV at different stages.