To　obtain　the　phase　change　heat　transfer　mechanism　of　molten　salt　in　single　energy　storage　tank,　quaternary　mixed　nitrate　with　low-melting-point　is　studied.　A　VOF　and　enthalpy-porosity　coupled　model　is　used　to　simulate　the　phase　change　process　of　phase　change　material　(PCM)　in　the　thermal　storage　unit　under　the　condition　of　bottom　heating,　which　is　verified　by　experiments.　The　results　show　that　the　obvious　temperature　stratification　and　a　fluctuation　of　the　solid-liquid　interface　can　be　observed　during　the　melting　process.　Heat　transfer　is　directly　affected　by　the　natural　convection　intensity,　and　the　position　of　the　solid-liquid　interface　determines　the　development　of　the　natural　convection.　The　results　in　this　paper　can　be　a　base　for　the　efficient　utilization　of　PCM　and　the　optimization　of　single-tank　thermal　storage　system.　©　2018,　Science　Press.　All　right　reserved.