In order to investigate the mode evolution law and vortex vibration generation mechanism of rectangular section flow field under vortexinduced vibration, twodimensional numerical simulation of vortexinduced vibration of rectangular section with aspect ratio of 5:1 was carried out based on Reynolds mean SST kω model.The CFD/CSD interleaved iterative solution combined with the dynamic grid technique was used to realize the twodimensional fluidstructure coupling calculation, and the wind tunnel test results were compared and analyzed to verify the accuracy of the numerical calculation. Then, the instantaneous state of the flow field was analyzed visually. Taking the vortex value in the z direction as the basic physical quantity, the static and vorticity modes of the flow field were compared and analyzed based on dynamic mode decomposition (DMD) method. The results show that the modal frequencies of the dominant flow field are different at different stages of the vortex vibration development. The frequency of the dominant mode changes from the static vortex devorticity frequency to the natural frequency of the structure, which represents the internal mechanism of the fluidstructure coupling. Compared with static rectangles, the phase difference between leading edge vortexes and trailing edge vortexes decreases due to the existence of motioninduced vortexes. When leading edge vortexes and trailing vortexes merge, the vortexes dominate. As a result, the correlation of Karman vortexes falling off at trailing edges is weakened, resulting in frequencylocking phenomenon of vortexes.