The invention relates to a silver/graphene-coated silicon dioxide composite upconversion nanocrystal and a preparation method thereof. The nanocrystal structure of the obtained silver/graphene@SiO2-NaLuF4:Yb,Gd,Er composite fluorescent nanoparticles is as follows: silver-on-graphene is used as an inner core and coated with silicon dioxide, and the upconversion nanocrystal is grafted onto the silicon dioxide surface. The metal reinforced fluorescence effect of the noble metal silver is utilized to enhance the luminescent intensity of the upconversion nanocrystal. The silver is supported on the graphene to well prevent the silver particles from aggregation, and the silver nanoparticles, which are dispersed and fixed to the graphene, generate field effect superposition due to short distance, so that the mutual polarity and surface plasma resonance are mutually reinforced. Since the graphene is an excellent electron acceptor and conductor, after the charges of the silver nanoparticles transfer to the graphene, the electron density becomes lower, the effective dielectric constant increases, and thus, the surface plasma resonance of the nano silver is enhanced, so that the metal reinforced fluorescence effect of the nano silver becomes higher. The method enhances the upconversion luminescent intensity of the rare-earth upconversion nanocrystal by about 50 times.