The invention relates to a nanometer composite material, in particular to a nanometer tin / carbon composite material and a preparation method thereof. A nanometer tin and carbon compound is obtained bytaking various carbon materials as a substrate, tin nitrate, tin chloride, tin sulfate, tin acetate, tin citrate and the like as a tin source, water containing an organic complexing agent, ethylene glycol, propylene glycol or other mixture as a solvent and sodium borohydride, potassium borohydride, hydrazine hydrate and the like as a reducing agent and by an absorption-decomposition-reduction method. According to the method, a solution containing tin ions is absorbed to a surface of a carbon material, remaining solution is filtered, the tin oxide / tin and carbon compound is obtained after thermal treatment, and the nanometer tin / carbon composite material is finally obtained by reduction reaction. In the composite material obtained by the method, metal tin particles are uniformly distributed in surfaces of carbon particles in nanometer size, and a phenomenon that a large amount of tin can be agglomerated by a traditional tin reduction method is prevented.