A high reversible capacity Si/C composite was synthesized by a thermal pyrolysis of citric acid embedded with fine graphite and silicon powders. The silicon was produced via an in-situ redox reaction between Mg and SiO. The structure, the morphology and the electrochemical performance of the materials were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), electrochemical tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared with the pristine silicon, the as-prepared composite exhibits much better electrochemical properties, which has a high reversible capacity of 780 mAh·g-1 at the 40th cycle with a retention up to 88.6%. The improved performance is mainly attributed to the uniform distribution of Si nanoparticles within the carbon matrix with buffering effect and excellent electric conductivity.