Sodium ion battery attracts significant attentions due to its unlimited sodium resources and low cost. However, sodium ion battery usually suffers from the cathode materials' low capacity and poor cyclic stability. Here we demonstrate that the ultrafine Cu7S4 nanoparticles as cathode for sodium ion battery, which is in-situ generated during the cycling of commercial sulfur/graphene composite electrodes, can enable superior specific capacity and ultra-stable cycling performance. The Cu7S4 has an open crystal structure for Na+ insertion, the graphene conductive networks ensure the fast transportation of electrons and also provide more active sites, and the ultrafine-sized Cu7S4 with a larger specific surface area guarantees efficiently electrochemical reactions. Thus, the ultrafine Cu7S4 nanoparticles and graphene nanosheets composite exhibit a high capacity of 585.5 mAh g−1 (0.41 mAh cm−2) at the current density of 100 mA g−1, ultra-stable cycling performance even after 2000 cycles with a capacity of 409.9 mAh g−1 (0.29 mAh cm−2) at a current density of 500 mA g−1 and excellent rate performance even at the highest current density of 2000 mA g−1 with a reversible capacity of 277.1 mAh g−1 (0.19 mAh cm−2).