Aiming at researching the effect of additional attack angle on the flutter performance of the triple-box girder， based on wind tunnel test of section model and theoretical calculation， additional attack angles under initial wind attack angles were obtained， and the critical flutter wind speed under the initial wind attack angle was calculated by Computational Fluid Dynamics （CFD） numerical simulation. Results indicated that the initial wind attack angle on the triple-box girder and transverse beams between the girders had a remarkable influence on the additional attack angle. Moreover， the section model encountered a significant additional attack angle effect when the initial wind attack angle was in the range of 0°~ +7°， and presented “hard flutter” since the torsional vibration amplitude increased rapidly with wind speed once flutter was triggered. It is difficult to directly obtain the precise critical flutter wind speed of the triple-box girder through a free vibration test on a section model. For suspension bridges with a main span of 3 300 m， the additional attack angle before flutter instability can reach above +7° with an initial wind attack angle of 0°， and lead to an obvious reduction of critical flutter speed. In other words， the aeroelastic stability cannot be fully utilized. Therefore， great attention should be paid to the additional attack angle effect of triple-box girders in the wind resistance design of super-long span bridges.