Abstract: The flutter performance of box girders was tested by wind tunnel experiment. The flutter process of a box girder was simulated according to a fluid-structure weak coupling calculation strategy and dynamic mesh technique. The movement of a solid model in a flow field was restricted by mesh size, and the grid large-deformation often leaded to calculation failure was solved by a layered-setting method of dynamic mesh spring constant. The formula of pneumatic energy inputting to the vibration model was put forward. The ratio of pneumatic energy inputting to different model-surface areas was also defined. From the viewpoint of energy balance, the flutter stability criterion was provided. The research found that the box girder windward side’s nozzle was the main pneumatic energy absorption area. In a complete oscillation cycle, the energy inputting to the vibration system by air was increased, leading to the loss of vibration stability. The flow field around the vibration section obtained by numerical simulation was analyzed by the phase-averaged method. The vortex evolution near the box girder tail wind nozzle was studied during flutter. The spatial distribution characteristics of box girder surface pressure were investigated by proper orthogonal decomposition technique. The results show that the main component of model surface pressure drift to the windward side nozzle in flutter.