The water-soluble Ag/Pt core-shell nanoparticles were prepared by deposition Pt over Ag colloidal seeds with the seed-growth method using K2PtCl4 with trisodium citrate as reduced agent. The Ag:Pt ratio is varied from 9:1 to 1:3 for synthesizing Pt shell layer of different thickness. A remarkable shift and broadening of Ag surface plasmon band around 410 nm was observed. The contrast of TEM images of Ag/Pt colloids has been obtained. Various techniques, such as transmission electron microscopy (TEM), UV-vis absorption and resonance light-scattering spectroscopy were used to characterize nanoparticles. The data of TEM, UV-vis and resonance light-scattering spectrum all confirm formation of Ag/Pt core-shell nanoparticles. Resonance light-scattering and emission spectrum show the Ag and Ag/Pt core-shell nanoparticles have a nonlinear light-scattering characteristic.