Influences of inclined web slope on flutter performance of flat box girders and their quantification were studied. Two sets of sectional models with fixed width and height, and different slopes of inclined web were designed and fabricated, dynamic parameters of the detecting system were kept unchanging, and flutter performances of different models were obtained with wind tunnel tests. The results showed that when the ratio of width to height for flat box girders is 11, effects of inclined web slope on the flutter critical wind speed are significant under the condition of 5 wind attack angle, but they are not obvious under the condition of 0 wind attack angle; when the ratio of width to height is 7, effects of inclined web slope on flutter are not obvious under either 5 wind attack angle or 0 one; changes of inclined web slope make flutter coupled derivative H3* and flutter direct derivative A2* vary significantly to cause greater variation of flutter critical wind speed. Finally, according test data and calculating results, adopting the flutter factor in the analytical calculation formula for the flutter critical wind speed, effects of wind attack angle inclined web slope on flat box girders’ flutter were quantified. Flutter factors under different conditions were deduced. It was shown that compared with the single reduced factor in the current flutter calculation code, conditioned flutter factors can be used to accurately calculate the flutter critical wind speed for flat box girders. The results provided a reference for effectively improving the aerodynamic layout of flat box girders.