The　interface　gap　between　track　slab　and　CA　mortar　layer　is　one　of　the　main　damage　modes　of　CRTS　II　slab　ballastless　track.　Based　on　the　analysis　of　the　gap　mechanism　between　track　slab　and　CA　mortar,　the　performance　function　of　the　gap　height　of　the　interface　between　track　slab　and　CA　mortar　layer　under　temperature　action　was　proposed.　Based　on　the　cohesive　zone　model　between　track　slab　and　CA　mortar,　a　finite　element　model　for　calculating　the　gap　height　of　the　interface　was　established.　The　first　four　moments　of　the　performance　function　were　evaluated　by　combining　the　univariate　dimension　reduction　based　point　estimation　method　with　the　finite　element　method,　with　the　reliability　index　being　calculated　using　the　high-order　moment　method.　The　influence　of　temperature　gradient　on　the　reliability　index　of　the　gap　height　of　the　interface　was　analyzed　under　different　interface　gap　forms.　The　results　show　that,　when　the　overall　temperature　rises,　the　reliability　index　of　the　gap　height　at　the　center　of　the　track　slab　decreases　with　the　increase　of　the　positive　temperature　gradient.　When　the　overall　temperature　drops,　the　reliability　index　of　the　gap　height　at　the　corner　of　the　track　slab　decreases　with　the　increase　of　the　negative　temperature　gradient.　The　probability　of　exceeding　the　gap　height　limit　is　high　under　extreme　temperature　conditions.　It　is　suggested　to　strengthen　the　maintenance　of　the　CRTS　II　slab　ballastless　track　structure.　©　2023　Science　Press.　All　rights　reserved.