Abstract: Thermal runaway is the most serious faults of lithium-ion battery, and there is still no effective method for on-line warning. Before the occurrence of the thermal runaway, a variety of gases of lithium-ion battery will separate out. According to the gas characteristics, the online warning of thermal runaway can be achieved. Firstly, the gas mechanism of the thermal runaway of lithium-ion battery is analyzed, thus the species of gas is determined. Subsequently, experiments are carried out based on the established platform to study the gas characteristics of the lithium-ion battery with different state-of-charge (SOC) before thermal runaway. Furthermore, based on volatile organic compounds (VOC), an online warning thermal runaway method is proposed for the lithium-ion battery with different SOC. Finally, the correctness of the proposed method is verified experimentally, and the results show that the warning quickness of the proposed method is significantly superior to that of conventional method. The studies on the gas characteristics before thermal runaway occurrence show that: (1) Carbon dioxide CO₂, carbon monoxide CO, methane CH₄, ethyl fluoride C₂H₅F and VOC are released at different SOC, while hydrogen H₂ is released when the SOC is higher than 10%; (2) With the increase of SOC, the volume fractions of VOC and H₂ continuously increase, while the volume fractions of CO first increases and then decreases, moreover, the volume fractions of CH₄ and C₂H₅F show no obvious changes; (3) Compared with CO₂, CO, H₂, CH₄ and C₂H₅F, VOC have the earliest releasedand the relatively high volume fractions are second to the fraction of CO₂. This work can provide important references for the research on the thermal runaway warning technology of lithium-ion battery.