Phase　transition　of　hydrogel,　which　is　polymerized　by　polymer　network,　can　be　regarded　as　the　transition　of　polymer　network　stability.　The　stability　of　the　polymer　network　might　be　changed　when　the　external　environment　changed.　This　change　will　lead　to　the　transformation　of　sensitive　hydrogels　stability,　thus　phase　transition　of　hydrogel　take　place.　Here,　we　present　a　new　free　density　energy　function,　which　considers　the　non-gaussianity　of　the　polymer　network,　chains　entanglement　and　functionality　of　junctions　through　adding　Gent　hyplastic　model　and　Edwards-Vilgis　slip-link　model　to　Flory-Huggins　theory.　A　program　to　calculate　the　phase　transition　temperature　was　written　based　on　new　free　energy　function.　Taking　PNIPAM　hydrogel　as　an　example,　the　effects　of　network　entanglement　on　the　phase　transition　temperature　of　hydrogel　were　studied　by　analyzing　the　microstructure　parameters　of　the　hydrogel　networks.　Analytical　results　suggest　a　significant　relationship　between　phase　transition　temperature　and　entanglement　network.