In the gap-ratio range of 0.0≤G≤7.0, a particle image velocimetry PIV is applied to conduct a systematic experimental research of the flow around a horizontal circular cylinder in the cross-flow of shallow water. The velocity distribution of transient flow field at various gap-ratios is obtained. Based on these data, the phenomena and rules of the vortex and its course of generation, development and evolvement at various gap-ratios are analyzed, and it is found that there are similar unshedding vortex structures at G = 0.0 and G = 7.0, and others are structures of shedding vortex. The figures of typical vortex movements are given. Based on this, the differences between the transient flow field and the time-averaged flow field and the characteristics of the vortex structures are analyzed. In addition when the Strouhal number keeps constant （about 0.2） concerning vortex shedding have been discussed. The findings of this paper are of guiding significance for engineering issues with similar flowing features.
