We investigated the effect of localized surface plasmon (LSP) on the efficiency of phosphors. Owing to the high densities of Au nanoparticles, we found that an additional absorption peak appeared at 650nm, which can resonant couple with red phosphors. To avoid agglomeration caused by high-density Au nanoparticles, we suggest using Au-SiO2 core-shell nanoparticles. The red phosphor efficiency was improved by achieving energy matching between the phosphor and LSP of the Au-SiO2 core-shell nanoparticles. By optimizing the Au nanoparticle density and spacer layer thickness, the red phosphor efficiency was improved by 18% as compared to that of the isolated red phosphor. This study provides a solution for fabricating white light emitting diodes with high efficiency, high color-rendering index, and wide color gamut.