Abstract: Flat box girders are widely encountered in Chinese high-speed railway (HSR) bridges, and the higher wind resistance of them is eagerly required for the running safety of high-speed trains. Therefore, it is necessary to perform refined investigations on the aerodynamics of the flat box bridge girder. Based on one of the HSR bridges in China, experimental and numerical investigations were both performed to study the effects of angle of bridge-girder faring θ on the aerodynamics of a flat box girder with B/D = 7 in wind angle of attack α = −12°, 12°. Experimental results shown that a drag fluctuations of the girder can be observed as θ = 30°, 45°, 52.5°(in practice), and 60°, i.e., the time-averaged drag coefficient decreases with an increase in α beyond the critical α_cr ≈ 5°. Among the tested range of θ, the decrease of \overline C _D caused by the drag fluctuations reach its maximum at θ = 60°. However, the drag fluctuations fade out with an increase in θ to 75°. Moreover, the effect of the girder asymmetry in the streamwise direction on the drag fluctuations is limited. Flow visualization and numerical simulation together have shown that the underlying flow mechanism of the drag fluctuations is the evolution of the flow separation bubble on the inclined web plate of the upstream bridge-girder fairing, namely, with an increase in α, the oncoming stagnation point moves downstream along the lower inclined web plate, resulting in a separation bubble and a strong negative suction on the upper inclined web plate, which finally leads to the drag fluctuations.