Abstract: The interference effect of aerodynamic forces of twin separate flat box girders were investigated by wind tunnel tests of sectional models. The interference-factor changing curves of aerodynamic coefficients versus separate distance were obtained. The research results show that the interference effect of drag coefficient of a forward box girder is slight. The interference effect of drag coefficient of a leeward box girder can be neglected when D/B≥2 and can be recognized as a reducing effect when D/B< 2. This reducing effect is more remarkable for a small distance. The interference effect of lift coefficient of a leeward box girder is slight. The interference effect of lift coefficient of a forward box girder can be neglected when D/B≥2 and can be recognized as a reducing effect when D/B< 2.