A theoretical model is developed to investigate the influence of external mean flow on sound transmission through an infinite double-leaf panel filled with porous sound absorptive materials. The sound transmission process in the porous material is described by using the equivalent fluid model, while fluid-structure coupling conditions are employed to ensure displacement continuity at fluid-structure interfaces. In order to verify the theoretical model, the model predictions are compared with existing experimental results. Numerical investigations are subsequently performed to quantify how a set of systematic parameters affect the sound transmission loss. It is demonstrated that the porous material affects the transmission loss in terms of both the absorption effect and the damping effect. Besides, thematerial loss factor and the thickness of the face plates also have an influence on the coincidence dip of the transmission loss curve. At frequencies below the coincidence frequency, the mean flow increases the transmission loss values due to the added damping effect of themean flow, whilst shifts the coincidence frequency upward because of the refraction effect of the mean flow. In addition, the coincidence frequency decreases with increasing azimuth angle between the sound incident direction and mean flow direction.