[en] In this paper, the fluorescence enhancement phenomenon of carbon quantum dots (C-QDs) via metal fluorescence effect has been investigated. Firstly, uniform Ag nanoparticles was prepared, which theoretically can supply a strong enhancement effect with a diameter as small as 20 nm. Then SiO₂ shell was utilized to control the distance between Ag core and C-QDs while surface charge of as-synthesized Ag@SiO₂ was further modified to bond C-QDs via electrostatic effect. Four different kinds of SiO₂ shell, with thickness 12 nm, 15 nm, 20 nm, and 27 nm, were synthesized to study the enhancement effect of distance. With the increase of SiO₂ shell, the fluorescence of Ag@SiO₂@NH₂-C-QDs nanocomposites rise first but then fall while the sample with the 20-nm shell obtained the best enhancement effect as high as 3.92-fold. Surface charge vibration of metal particles was used to explain this phenomenon.