Abstract: The flutter performance of closed box section was tested by wind tunnel experiment. Based on flowsolid loose coupling calculation strategy and dynamic grid technology, the flutter process of box girder was simulated by computational fluid dynamics (CFD) method. The evolution of vortex near the rear nozzle of the model in the flutter critical state was studied using the method of phaseaverage. The results indicated that the alternation of up and down vortex near the model rear nozzle had stronger effect on structure periodic vibration. The features of energy input to the model surface different areas during the flutter were researched by adopt of block analysis. The influence of evolution role of vortex near the model tail on the model surface aerodynamic spatial distribution and the airflow energy input characteristics were also studied. The research show that when the model was in critical flutter state, the windward side nozzle absorbed a lot of energy from the air and in a complete vibration cycle, the energy input to the vibration of the system increased, and the result is the sudden loss of structural stability.