Composite　cooling　is　a　promising　cooling　technique　in　modern　gas　turbines.　Quantifying　the　contributions　of　its　internal　and　external　cooling　will　help　improve　the　cooling　design　and　increase　the　cooling　effectiveness.　In　this　paper,　a　conjugate　heat　transfer　(CHT)　decoupled　method　is　established　firstly　to　decouple　the　complex　interaction　between　internal　and　external　cooling　for　a　composite　cooling　structure.　And　then　the　rationality　of　the　decoupled　method　is　verified　by　comparing　the　flow　and　heat　transfer　characteristics　of　the　decoupled　model　and　the　original　composite　cooling　model.　Using　the　decoupled　model,　the　contributions　of　the　internal　and　external　cooling　on　overall　cooling　effectiveness　are　quantitatively　evaluated,　and　the　external　cooling　is　found　to　encumber　the　overall　cooling　effectiveness　when　the　film　flow　badly　lifts　off　from　the　external　surface.　The　detailed　contributions　of　external　cooling,　impingement　cooling,　and　film　hole　convective　cooling　on　overall　cooling　effectiveness　of　the　research　model　are　additionally　illustrated.　Moreover,　a　prediction　method　is　presented　to　estimate　the　overall　cooling　effectiveness　based　on　the　decoupled　model,　and　a　small　prediction　error　is　verified.　The　methods　presented　in　this　paper　could　be　used　in　the　preliminary　and　optimization　stages　of　the　hot　end　component　design　for　gas　turbines.　©　2019　Elsevier　Ltd