Abstract: The flow characteristics around an H-shape bridge deck in its flutter was studied by the computational fluid dynamics (CFD) numerical method. The effect of the vortex evolution on the model different region’s aerodynamic force and pneumatic energy was researched by block analysis. The research shows that the vortex evolution caused by the changeable characteristics of the surface pressure on the model area near the windward side platen had strong regularity. While the change of pressure on the model region near the leeward side baffle had no obvious rule. Block analysis show that the model up side surface areas near the windward side platen absorbed the air energy, and the air energy input to the model surface near the leeward side platen was fluctuate. Thusly, the total air energy input to the model was also fluctuate, which caused a long time larger amplitude of torsional vibration of H-shape bridge decks in the flutter.