Colloidal　nanocrystal　scintillators　hold　great　potential　in　fabricating　large-area,　flexible　X-ray　detectors　for　highresolution　X-ray　imaging　of　highly　curved,　irregularly　shaped　objects.　The　synthesis　of　high-efficiency,　highstability　nanoscintillators　is　of　great　importance　for　the　development　of　X-ray　imaging　detectors.　In　this　study,　we　develop　a　class　of　cerium　(Ce3+)-sensitized　core-shell　nanoscintillators　that　are　suitable　for　achieving　flexible　Xray　luminescence　imaging.　We　demonstrate　that　an　epitaxial　growth　of　NaGdF4:Ce(60%)　on　the　surface　of　NaGdF4:Eu(15%)　nanoscintillators　as　a　sensitization　layer　allows　for　enhancing　X-ray-induced　radioluminescence.　We　reveal　that　the　enhancement　of　X-ray　luminescence　in　nanoscintillators　could　be　attributed　to　the　synergistic　effect　of　high-Z　composition-induced　X-ray　absorption,　Ce3+　sensitization,　and　surface　passivation　to　relieve　energy　quenching.　By　incorporating　the　nanoscintillators　into　a　flexible　elastomer　of　polydimethylsiloxane　(PDMS),　we　demonstrate　its　utility　in　high-resolution　flexible　X-ray　luminescence　imaging.