Engineering　a　functional　nanoplatform　that　integrates　dynamic　monitoring　of　endogenous　biomarkers　and　a　stimuli-activated　therapeutic　mode　is　promising　for　early　diagnosis　and　treatment　of　cancers.　In　this　study,　we　developed　an　intelligent　DNA　nanohydrogel　with　specific　targeting　capability　that　can　be　stimuli-activated　for　both　in　vitro　telomerase　detection　and　in　vivo　telomerase-triggered　gene　therapy.　The　DNA　nanohydrogel　was　formed　simply　by　the　self-assembly　of　two　Y-shaped　DNA　units　and　a　double-stranded　DNA　linker　labeled　with　fluorophores　and　loaded　with　therapeutic　siRNA.　When　intracellular　telomerase　was　overexpressed,　the　DNA　nanohydrogel　collapsed　owing　to　the　prolongation　of　the　telomeric　primer　at　the　terminal　sequence　of　one　of　the　Y-shaped　DNA　units.　As　a　result,　the　quenched　fluorescence　due　to　fluorescence　resonance　energy　transfer　(FRET)　of　the　DNA　nanohydrogel　recovered　and　the　trapped　siRNA　was　released,　enabling　the　accurate　detection　and　imaging　of　intracellular　telomerase　activity　as　well　as　effective　gene　therapy　of　tumors.　Benefiting　from　the　great　biocompatibility,　specificity,　and　stimuli-responsive　property,　the　developed　DNA　nanoplatform　provides　a　new　opportunity　for　precise　cancer　diagnosis　and　treatment　as　well　as　other　biological　applications.