Lanthanide-doped　upconversion　nanoparticles　(UCNPs)have　potential　applications　in　optical　and　optoelectronic　devices　due　to　their　novel　optical　properties.　The　surface　plasmon　is　an　effective　method　in　improving　the　upconversion　luminescence　(UCL)　properties,　and　it　is　essential　to　tunee　the　distance　between　the　metal　NPs　and　UCNPs　for　improving　UCL.　In　this　work,　the　Ag/TiO2/NaGdF4:Er3+/Yb3+/Al3+　composite　films　were　fabricated,　and　the　effect　of　the　TiO2　layer　thickness　on　spectral　characteristics　in　the　composite　films　was　investigated.　It　is　shown　that　the　thickness　of　TiO2　film　has　a　significant　effect　on　the　Ag　NPs　plasma　resonance　and　fluorescence　intensity.　The　maximum　upconversion　emission　enhancement　factors　of　the　red　and　green　emission　were　obtained　for　the　sample　with　15-nm-thickness　TiO2　under　the　980 nm　excitation.　That　is　mainly　originated　from　the　enhancement　of　the　resonate　energy　transfer　(Yb3+→Er3+)　rate　under　the　competition　between　non-radiative　transition　and　local　field　effects.　©　2022　Informa　UK　Limited,　trading　as　Taylor　&　Francis　Group.