The　reactor　containment　integrity　and　the　pressure　inside　the　containment　are　the　most　concern　in　the　practical　running　process.　To　evaluate　the　transient　heat　transfer,　non-equilibrium　fraction　(NEF)　is　proposed　to　represent　the　completed　degree　of　transient　condensation.　An　empirical　correlation　for　NEF　is　developed　and　the　fit　error　is　within　±25%　for　97%　of　results.　Based　on　this,　a　simplified　calculation　model　for　transient　heat　transfer　rate　is　also　developed,　then　the　transient　condensation　heat　transfer　coefficient　(HTC)　is　obtained　to　characterize　the　intensity　of　transient　heat　transfer.　The　effect　of　mixed　gas　pressure,　air　mass　fraction　and　temperature　of　cooling　water　on　NEF　and　transient　condensation　HTC　are　discussed.　The　result　indicates　that　the　initial　air　mass　fraction　and　initial　gas-mixture　pressure　play　an　important　role　in　transient-state　heat　transfer,　and　the　influence　of　initial　cooling　water　temperature　can　be　ignored.　This　study　aims　to　improve　the　understanding　of　transient-state　heat　transfer　and　provide　guidance　to　practical　engineering.　©　2022　Elsevier　Masson　SAS