Lanthanide　(Ln(3+))　oxysulfide　nanocrystals　(NCs)　have　great　prospect　in　many　advanced　technologies;　however,　they　suffer　from　a　low　photoluminescence　efficiency　due　to　the　volatility　of　sulfur　and　deleterious　surface　quenching　effect.　Herein,　we　report　a　novel　sandwiched　luminescent　heterostructure　based　on　Ln(3+)-doped　Gd2O2S@NaYF4　core/shell　NCs　with　tunable　sulfur　content　in　the　sandwich　layer.　By　means　of　Eu3+　as　the　sensitive　structural　probes,　we　unravel　the　ligand-mediated　structure　control　of　the　NCs　from　Gd2O3:　Ln(3+)@NaYF4　to　Gd2O2S:　Ln(3+)@NaYF4　with　tailored　S2-　deficiency.　Such　a　sandwich-type　core/shell　heterostructure　enables　us　to　achieve　efficient　and　multicolor　downshifting　and　upconversion　luminescence　(UCL),　with　up　to　208.8　folds　of　enhancement　in　UCL　intensity　as　compared　to　that　of　their　core-only　counterparts.　These　findings　provide　a　general　approach　for　the　controlled　synthesis　of　lanthanide　oxysulfide@fluoride　heterostructure,　which　offers　a　new　way　for　the　materials　design　towards　diverse　emerging　applications.