As a promising energy storage system, the sodium-ion battery has attracted wide attention due to its rich sodium resources and cost advantages in energy storage and low-speed electronic vehicle application. With the development of the sodium-ion battery industry, battery safety is one of the key issues which is often caused by the heat loss of battery cells. In this work, the safety factors of sodium-ion batteries were studied, including thermal stability of hard carbon materials, over-discharge test, safety test (extrusion and acupuncture, etc.), and thermal runaway test. According to the first cycle discharge curve of a hard carbon coin cell, the solid electrolyte interphase (SEI) of hard carbon in different discharge potentials with differential scanning calorimetry (DSC) at 0.9, 0.5, 0.15 and 0.01 V, respectively were studied. The results showed that with the increase of sodium embedded in the hard carbon, the position of the exothermic peak appeared earlier and became more obvious and the presence of electrolytes reduces the stability of hard carbon embedded with sodium. The safety performance of full sodium-ion batteries can be evaluated by calorimetric analysis of the thermal runaway process of batteries. 1 Ah soft package batteries were prepared to study the over-discharge test and safety test. Compared with the non-over-discharge cells, cells over-discharge to 0 V had little difference in cycle performance after 500 cycles. The current density (0.1 or 1 C) had no significant influence on capacity recovery and cycle performance of the battery. The extrusion and acupuncture tests showed that the sodium-ion battery had good safety performance as the batteries were no fire and no explode. In addition, the thermal runaway test as calorimetric analysis was generally carried out by an accelerating rate calorimeter (ARC). The ARC test showed that the onset temperature of detectable self-heating were 136.6, 131.6, 136.3, 128.2, 166.6 and 138.6℃ at 80% state of charge (SOC), 60% SOC, 50% SOC, 40% SOC, 30% SOC and 0% SOC, respectively. Moreover, the thermal runaway occurred at 240.9℃ only at 80% SOC. It proved that the safety performance of the sodium-ion battery was good and the sodium-ion battery had the best safety performance at 30% SOC.