Novel Ag@C core-shell structural nanospheres were prepared with Au nanoparticles decorated on their surface. The Ag@C nanospheres were first synthesized by a hydrothermal method, and then Au nanoparticles were absorbed onto the surface of the prepared nanospheres via electrostatic effect. In the study of its electrochemical behavior using hemoglobin (Hb) as a model protein, the prepared nanocomposite displayed excellent biocompatibility and high electrical conductivity. When it was used to fabricate a biosensor for H2O2 detection, excellent electrocatalytic activity was observed. This study may offer a new approach to expand the syntheses and electrochemical applications of core-shell structural nanospherical composites.