Molten　salts　are　applied　in　solar　thermal　power　plant　as　heat　transfer　and　heat　storage　medium.　In　this　work,　the　thermal　stability　of　quaternary　nitrate-nitrite　(QNN)　mixed　salt　and　Solar　salt　are　studied　under　three　constant　temperature　conditions　(500　degrees　C,　565　degrees　C,　600　degrees　C)　for　1008h.　The　results　show　that　the　mass　ratio　of　NO2-1/NO3-1　plays　an　important　role　in　the　thermal　physical　properties　of　nitrite/nitrate　molten　salt.　The　mass　ratio　of　NO2-1/NO3-1　in　Solar　salt　increases　with　the　increasing　of　working　temperature,　which　results　in　the　decreasing　of　melting　point　and　initial　crystal　temperature　and　the　increasing　of　decomposition　temperature.　The　average　values　of　melting　point　and　initial　crystal　temperature　of　QNN　mixed　salt　are　maximum　119　degrees　C　and　24　degrees　C　lower　than　that　of　Solar　salt.　Meanwhile,　the　maximum　increase　of　average　values　of　specific　heat　and　thermal　conductivity　of　QNN　mixed　salt　are　14.26%,　8.18%　compared　with　its　base　salt,　respectively.　When　the　working　temperature　is　higher　than　decomposition　temperature,　average　values　of　specific　heat　and　thermal　conductivity　of　Solar　salt　decrease　because　of　the　acceleration　of　nitrite　formation.　The　viscosity　values　of　QNN　mixed　salt　and　Solar　salt　decrease　with　the　increasing　of　temperature.　Based　on　the　comparative　analysis,　QNN　mixed　salt　has　good　application　value　in　practical　engineering　applications.