In　this　work,　a　ligand-free　luminescent　hydrogel　is　used　as　a　visual　ratiometric　fluorescence　sensor　to　detect　rareearth　(RE)　ions　for　the　first　time.　This　hydrogel　is　easy　to　be　prepared,　environmentally　friendly,　and　has　high　luminescent　efficiency.　It　is　found　that　the　fluorescence　intensity　and　emission　color　of　the　hydrogel　immediately　change　significantly　when　solution　of　active　RE3+　3　+　ions　is　added　without　any　treatment.　According　to　the　linear　relationship　between　the　fluorescence　intensity　ratio　and　the　concentration　of　RE3+　3　+　solutions,　the　detection　limit　is　calculated　to　be　as　low　as　0.94　mu　M.　The　detection　of　RE　ions　has　strong　anti-interference　to　other　ions.　Therefore,　this　sensor　can　be　used　for　preliminary　field　exploration　of　RE　ores　and　detection　of　waste　containing　small　amounts　of　RE　ions.