Na-ion batteries are considered as promising alternatives for Li-ion batteries in electrochemical energy-storage. However, the lack of high performance anode materials severely plagues their practical application. In this work, we synthesized the SnO2@graphene (SnO2@G) nanocomposites through one-pot hydrothermal method, in which the ultrafine SnO2 nanoparticles (∼4 nm) evenly distributed on the graphene sheets surface. The synthesized SnO2@G delivered a Na storage capacity of 343 mAh g−1 at 100 mA g−1 after 100 cycles, with excellent capacity retention. Even at a low temperature of −20 °C, SnO2@G still maintains a specific capacity of 97 mAh g−1 after 100 cycles, making it both available for ambient and low temperature environment. The detailed Na-storage mechanism for SnO2@G is revealed. It is found that both conversion and alloying reaction contribute to sodium storage. The dominating contribution from conversion reaction is attributed to the ultrafine nanoparticles, which triggers the activity of conversion reaction of SnO2 with sodium. The study provides new insights for using SnO2 as the anode materials for Na-ion batteries.