Owing　to　the　possibility　of　controlling　its　specific　mechanical　behaviors　uptaken　by　irradiated　by　light　at　particular　wavelengths,　the　photoisomerization　hydrogels　have　a　broad　range　of　potential　applications.　A　theory　connecting　the　optical　excitation　to　mechanical　behavior　is　essential　to　precisely　control　the　photo-mechanical　behaviors　of　the　hydrogel.　In　this　work,　a　photo-mechanical　coupling　theory　is　developed　to　describe　the　photo-mechanical　responses　of　photoisomerization　hydrogels　within　the　framework　of　finite　deformation　continuum　thermodynamics.　In　the　model,　the　deformation　gradient　is　decomposed　into　two　parts　to　effectively　model　the　light-induced　deformation　and　the　elastic　one.　To　consider　the　effect　of　the　optical　excitation　on　mechanical　behaviors,　we　first　investigate　the　transporting　mechanism　of　light　in　hydrogel,　as　well　as　the　photochemical　reaction　process;　and　we　then　explore　the　disturbance　of　light　irradiation　on　the　equilibrium　of　the　thermodynamic　system　of　hydrogel,　as　well　as　the　relationship　of　conformational　entropy　of　hydrogel　network　with　the　photochemical　reaction;　finally,　based　on　the　entropy　elasticity　theory,　we　propose　a　new　free　energy　function　of　the　photosensitive　hydrogel　to　consider　the　effect　of　molecular　chain　distribution　evolution　on　the　stiffness　of　the　hydrogel　network.　With　the　implementation　of　the　proposed　model,　we　study　the　photo-mechanical　behaviors　and　mechanical　properties　of　photoisomerization　hydrogels.　The　present　research　is　helpful　for　understanding　the　multi-field　coupling　behaviors　of　the　photosensitive　hydrogel,　and　then　providing　guidelines　for　the　application　of　photoisomerization　hydrogel.　©　2023　Elsevier　Ltd