The　stability　of　hydrogels　can　be　considered　as　the　stability　of　polymer　networks　of　the　gels.　The　external　environment　can　lead　to　the　change　of　the　polymer　networks　stability　and　the　phase　transition　of　hydrogels　occurs.　In　this　paper,　a　new　free　density　energy　function　considering　the　chain　entanglements　and　functionality　of　junctions　is　presented　via　combining　Gent　hyperelastic　model　and　Edwards-Vilgis　slip-link　model　with　Flory-Huggins　theory.　A　numerical　method　is　developed　to　solve　the　phase　transition　temperature　of　the　hydrogels　based　on　the　given　new　free　energy　function.　Influences　of　four　microstructure　parameters　on　phase　transition　temperature　of　hydrogels　are　investigated　by　taking　PNIPAM　hydrogel　as　an　example.　Analytical　results　show　that　the　microstructure　parameters　might　affect　the　phase　transition　of　the　hydrogels　significantly.　©　Published　under　licence　by　IOP　Publishing　Ltd.