Abstract: The experimental study on thermal runaway of lithium-ion battery caused by nail-penetration is conducted, based on which the evolution is characterized as four main stages, namely the structure damage, direct internal short circuit(ISC), wide range ISC, and delamination stage. The battery voltage and temperature variation during thermal runaway is analyzed. It is found that there exist two extremums of temperature rise rate, which are in accordance with the stage two and stage three, respectively. The period around first extremum is the key point for restraining thermal runaway. The electrothermal model is established and the impacts of battery state of charge(SOC) and cooling condition are disclosed, indicating that increasing heat transfer coefficient mainly increases has a weak effect on suppressing TR for high SOC batteries, and its effect is mainly reflected in increasing the temperature difference between the inside and outside of the battery. However, for medium and low SOC batteries, increasing the heat transfer coefficient can significantly suppress thermal runaway. Therefore, quickly discharging high-risk batteries to a low SOC state is an effective method to suppress thermal runaway. The conclusions are potential to be useful in further diagnosis, prediction and safety management design of the lithium-ion batteries.