Ln-containing　polyoxoniobates　(PONbs)　have　appealing　applications　in　luminescence,　information　encryption　and　magnetic　fields,　but　the　synthesis　of　PONbs　containing　high-nuclearity　Ln-O　clusters　is　challenging　due　to　the　easy　hydrolysis　of　Ln3+　ions　in　alkaline　environments.　In　this　paper,　we　are　able　to　integrate　CO32−　and　high-nuclearity　Ln-O　clusters　into　PONb　to　construct　an　inorganic　giant　Eu19-embedded　PONb　H49K16Na13(H2O)63[Eu21O2(OH)7(H2O)5(Nb7O22)10(Nb2O6)2(CO3)18]·91H2O　(1),　which　contains　the　highest　nuclearity　Eu-O　clusters　and　the　largest　number　of　Eu3+　ions　among　PONbs.　In　addition,　the　film　that　was　prepared　by　mixing　1　with　gelatin　and　glycerol,　exhibits　reversible　luminescence　switching　behavior　under　acid/alkali　stimulation　and　has　been　used　to　create　a　fluorescence-encoded　information　approach.　This　work　paves　a　feasible　strategy　for　the　construction　of　high-nuclearity　Ln-O　cluster-containing　PONbs　and　the　expansion　of　the　application　of　Ln-containing　PONbs　in　information　encryption.　©　2024