Coupling　and　interaction　of　multi-physical　fields　exist　in　hydrogel　consisting　of　a　fluid　and　a　solid　under　external　stimulus.　In　this　paper,　a　phenomenological　theory　for　chemo-mechanical　coupling　behavior　and　finite　element　formulation　are　developed,　based　on　the　thermodynamic　laws.　The　free　energy　function　is　constructed　and　used　to　derive　the　constitutive　equations　and　governing　equations　for　a　linear　coupling　system　including　a　chemical　effect.　Equivalent　integral　forms　of　the　governing　equations　and　coupled　finite　element　equations　are　obtained　by　a　variational　principle.　Numerical　examples　demonstrate　the　interaction　of　chemical　and　mechanical　effects　of　hydrogel　under　external　force　loadings　and　chemical　stimuli.　It　is　shown　that　the　chemo-mechanical　coupling　behavior　of　hydrogel　can　be　described　by　the　theory　and　numerical　method　presented　in　this　paper.