The fluctuations in the unsteady flow channel field can easily affect the efficiency of mass transfer and heat transfer.An experimental system is constructed to carry out 2DPIV measurement of the transient velocity vector field flow of unsteady field. With the experimental system, and the forms of vector field and vorticity distribution at each time are analyzed, and the cycle and frequency of the unsteady flow are obtained. The transient velocity vector field of unsteady flow field is decomposed using Proper Orthogonal Decomposition (POD) technology. According to the energy ratio of various modes , the first 16 modes as dominant modes are selected to represent dominant structure of unsteady flow field, and the first 16 modes coefficient of the POD decomposition are analyzed using power spectrum. The decomposition results show that the low order modes with POD are the fundamental frequency and corresponding frequency multiplication of the unsteady flow field, and the fluctuation mechanism of unsteady flow field is revealed. The fluctuation of the unsteady flow field is caused by some vortexes and cosines wave formed mainstream, the fluctuation frequency of the vortexes and wave mapping of the mainstream maps the dominant low order coefficients of POD.