To date, there has been much research on electrochemical study of lithium ion batteries. However, the question has also been raised for depletion of lithium resources, causing interest in Na-ion batteries (NIBs). While extensive investigations on various transition metal oxides and chalcogenides as an anode material for NIBs have been carried out, few of them have been able to utilize their high specific capacity in sodium-based systems. Here, the mixed Sn-S nanocomposites on reduced graphene oxide are prepared via a facile hydrothermal synthesis and a unique carbo-thermal reduction process, producing ultrafine nanoparticle with a size of under 2 nm. It has been confirmed to overcome the intrinsic properties of tin sulfides such as large volume change and sluggish diffusion kinetics by experimental results, which demonstrating an outstanding electrochemical performance; excellent specific capacity of 1230 mAh g^-1, impressive rate capability (445 mAh g^-1 at 5000 mA g^-1) and cycling stability. The electrochemical behaviors of conversion-alloying reactions for the anode materials are also investigated revealing both the structural transition and the chemical state in the discharge/charge process. Comprehension of the reaction mechanism for the mixed Sn-S/rGO hybrid nanocomposites makes it a promising electrode material and provide a new perspective on the anode field for Na ion batteries.