The separated and reattaching turbulent flow over a surface-mounted two-dimensional square cylinder was experimentally studied by using time-resolved particle image velocimetry (TR-PIV). A total of 61,440 instantaneous image frames were acquired at a framing rate of 125 Hz, yielding a reliable result of the statistical quantities. The time-averaged features of the separated and reattaching flow were analyzed in terms of distributions of the velocity vectors, vorticity, the streamwise velocity fluctuation intensity and shear stress. The association between the large-scale vortical structures and spatial variation of these time-averaged quantities were thoroughly discussed. The unsteady features of the flow were revealed from distributions of the reverse-flow intermittency, space-time contour plot of the fluctuating streamwise velocity, and cross-correlation of the streamwise velocity. Subsequently, a comprehensive understanding of the contribution of the flow structures into the fluctuating flow field was gained by using a snapshot proper orthogonal decomposition (POD) analysis. The results showed that the linear combination of the first five POD modes, which capture 57% of the fluctuation energy, was capable of representing the large-scale behaviors of the separated and reattaching turbulent flow in the senses of spectrum, instantaneous feature and spatial variation of the velocity fluctuation intensity.