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In this study, a facile and nontoxic synthetic method of well-covered Cu-Ag core-shell nanoparticles (Cu@Ag NPs) was determined. Through chemical reduction reaction in a modified silver ammonia solution with gelatin additive, the as-synthesized Cu@Ag NPs showed attractive comprehensive characteristics, including small and narrow size distribution, low proportion of Ag, dense and uniform Ag shell, good dispersibility, good sphericity, and desirable oxidation resistance. Based on electrochemical analysis, a complexing model was proposed to explain the effects of gelatin on the deposition of Ag. It revealed that the gelatin acted as not only the steric hindrance but also the complexing agent and diffusivity modifier. The gelatin addition could increase the stability and decrease the decoupling rate of complexed Ag ions, thus leading to significant negative shift of the deposition potential and decrease of the deposition rate, promoting the formation of finer and thinner Ag shell. Our study proposes an applicable approach for the synthesis of Cu@Ag core-shell nanoparticles, and the complexing model can provide a theoretical tool to describe the effects of complexing agent on the synthesis of Cu@Ag and other bimetallic nanoparticles. [Display omitted] • Well-covered Cu@Ag nanoparticles were synthesized by a facile one-step method. • Desirable morphology, coating quality and oxidation resistance were achieved. • Complexing effect is the key controlling the formation of fine and thin Ag shell. • Gelatin additive increases the complex stability and decrease the decoupling rate. • A complexing model was proposed and the prediction consisted well with experiments. [ABSTRACT FROM AUTHOR]