Abstract: Time-resolved particle image velocimetry (PIV) with sampling frequency f=1000Hz was used in 0.55m×0.4m aeroacoustic wind tunnel to test the unsteady characteristics of the flow behind a D=20mm diameter circular cylinder in an area of 7.5 times the diameter long and 6.6 times the diameter wide at Reynolds numbers of 2.74×10⁴.The characteristics of the mean and pulsating flow fields, as well as the frequency characteristics of the cylinder's wake have been revealed.A phase-averaged analysis method based on the cross-correlation coefficients among the PIV velocity vectors has been proposed to capture the process of vortex production, shedding, development and dissipation.The results indicate that there exists a low-speed recirculated flow area behind the circular cylinder, where the flow structure changes most intensively.The vortices shed from the upper and lower sides of the circular cylinder cross and correlate around the 1.9D downstream area, where the turbulence is the strongest.Strouhal number corresponding to the vortex shedding frequency of the cylinder's wake is stable around 0.2.The proposed phase-averaged analysis method is simple but efficient to study the spatial-temporal evolutions of the vortex shedding, which can be easily applied to other areas in unsteady flow field measurement.