Abstract: Structures with square cross-sections are widely used in engineering, thusly it is important to investigate the wind loading and wind-induced vibration characteristics of the flow around one square cylinder. The unsteady flow field of flow around the square cylinder usually involve a variety of complex flow phenomena. It is significant to capture the main modes of flow for the analysis of the flow field characteristics and the control of flow. In this study, a Reduced Order Model (ROM) is established upon the Dynamic Mode Decomposition (DMD) method to extract the main modes of the flow field and to reconstruct the flow field. The snapshots of the unsteady flow field are obtained upon numerical simulations. The main modes are extracted using DMD. The extracted modes are utilized to reconstruct the unsteady flow field. The results show that the first fourth modes extracted by the DMD method can accurately reconstruct the original flow field and provide a reference for flow control and structure design.