A mathematical model is constructed to analyze velocity flow filed of windtunnel test using POD method. Based on this model, the instantaneous velocity field at the leeward side of a bump is analyzed. The results show that the first few POD modes occupying most of the energy represent the largescale coherent structures that dominate the global flow field while higher POD modes occupying less energy represent the smallscale structures and background noise. The reconstructed velocity field by the first four POD modes can exclude smallscale flow structures, containing only largescale structures, which researchers are interested in.