Lanthanide　metal　halide　double　perovskites　(Ln-MHDPs)　are　promising　scintillator　materials　due　to　their　outstanding　luminescence　properties　and　high　X-ray　absorption　cross-sections.　With　the　growing　demand　for　advanced　X-ray　imaging　scintillators,　there　is　increasing　interest　in　the　exploration　of　new　scintillation　materials　suitable　for　novel　applications　under　harsh　conditions.　In　this　study,　we　present　a　series　of　Ln-MHDPs　with　tunable　fluorescence　emission　spanning　the　visible　and　near-infrared　regions,　effectively　activated　by　both　ultraviolet　and　X-ray　excitation.　The　Cs2NaLnCl6　(Ln　=　Eu,　Tb,　Ho,　Er,　and　Yb)　compounds　demonstrated　high　X-ray　sensitivity,　excellent　stability,　and　low　detection　limits　under　X-ray　irradiation.　Notably,　Cs2NaTbCl6　exhibited　a　light　yield　of　42　200　photons　per　MeV,　while　retaining　98%　of　its　initial　radioluminescence　intensity　after　180　minutes　of　X-ray　exposure,　corresponding　to　a　total　radiation　dose　of　approximately　86.4　Gy.　Moreover,　these　materials　achieved　a　spatial　resolution　of　10　lp　mm-1,　even　in　flexible　forms.　The　application　of　polydimethylsiloxane　coating　significantly　enhanced　the　water　resistance　of　the　Cs2NaLnCl6　compounds　and　facilitated　clear　underwater　X-ray　imaging.　This　study　highlights　the　promising　potential　of　Ln-MHDPs　for　underwater　X-ray　imaging　applications.