Abstract: Flow around a slit circular cylinder was investigated in a water tunnel with particle image velocimetry(PIV) technique, power spectrum analysis, proper orthogonal decomposition(POD) method and other techniques for data processing. The experimental results show that the flow in the slit demonstrated periodic motion. This oscillating vent-flow has greatly altered the near wake flow features within a certain range of Reynolds number. The total turbulence kinetic energy(TKE) almost contributes from the first six modes. The first and second POD modes control the vortex shedding alternately process. The third and fourth POD modes control the process of vortex energy transition along the flow direction. The interaction of co-rotating vortex pair in the fifth and sixth POD modes effects the vortex energy during the vortex shedding process.