A　theoretical　model　is　developed　to　investigate　the　influence　of　external　mean　flow　on　sound　transmission　loss　(STL)　through　composite　sandwich　panels　filled　with　porous　materials.　The　equations　of　the　motions　for　the　composite　laminate　plates　are　established　by　adopting　the　first-order　shear　deformation　theory.　The　porous　materials　in　between　the　sandwich　structures　are　characterized　by　an　equivalent　fluid　model.　Employing　the　fluid-structure　coupling　condition,　the　displacement　continuity　at　fluid-structure　interfaces　is　guaranteed.　To　validate　the　proposed　theoretical　model,　a　comparison　among　the　present　theoretical　predictions,　the　existing　theoretical　and　experimental　results　are　executed,　with　overall　agreements　achieved.　A　systematical　investigation　is　carried　out　to　explore　the　influences　of　the　external　mean　flow,　incidence　angle　and　laminate　scheme　on　the　coincidence　frequency　and　sound　transmission　loss　of　the　sandwich　structures.　This　work　provides　a　helpful　guideline　for　the　vibroacoustic　design　of　composite　sandwich　structures.　©　2021