To increase emission of up-converting materials, NaGdF₄:20%Yb³⁺/2%Er³⁺@NaGdF₄@Ag up-converting nanoparticles (UCNPs) in core–spacer–shell structure have been designed and fabricated. Significant emission enhancement compared with ordinary NaGdF₄:20%Yb³⁺/2%Er³⁺ has been obtained. Furthermore, the enhancement mechanism have been found as the protective effect of NaGdF₄ and localized surface plasmon resonance (LSPR) of Ag.