Abstract: With the rapid development of electrochemical energy storage projects, the safety of energy storage battery has attracted more and more attention. In order to prevent and control the thermal runaway risk of lithium-ion battery, it is necessary to clarify the behavior of thermal runaway. Based on the lithium-ion battery thermal runaway experimental platform, overheating is used to trigger the thermal runaway of 86 Ah iron phosphate lithium battery. The process of thermal runaway heat and gas generation is analyzed. The thermal runaway process is divided into three stages by two critical peaks at the pressure relief valve opening and maximum temperature. In the process of thermal runaway, it is found that there are two peaks of temperature rise rate corresponding to 110 ℃ and 225 ℃, and the possible reasons for the temperature rise rate peaks are further analyzed. Moreover, the online detection of gases measuring is conducted through Fourier Transform Infrared (FTIR) and hydrogen probe. The real time components of the gas produced by thermal runaway are obtained and various gas generation reactions are summarized in detail. Finally, the thermal runaway gases are integrated during the whole thermal runaway process. The results reveal that carbon dioxide and hydrogen are main components in the gas production and account for 30.15% and 39.5%, respectively. The results can provide insight into the early warning of lithium-ion battery fire accident in electrochemical energy storage.