Abstract: The thermal runaway warning of lithium iron phosphate battery for energy storage is an urgent problem waiting to be solved in large-scale application. Firstly, the mechanism of thermal runaway gas production of lithium iron phosphate batteries is analyzed, and hard-shell lithium iron phosphate battery(composed of 32 single cells in four-in-eight series) and soft-packed lithium iron phosphate battery(composed of 72 single cells in six-in-twelve series) are used as test objects. The test platform is consistent with the real environment of energy storage cabin. The whole process of overcharge of hard shell and soft pack lithium iron phosphate battery module to thermal runaway is studied by constant current overcharge method. The visible light camera, gas detector, and infrared monitoring system are used for all-round on-line monitoring of gas production. The experimental results show that the overcharge to thermal runaway combustion of lithium iron phosphate batteries is not a sudden change but a gradual process. There are differences between hard-shell lithium iron phosphate batteries and soft-packed lithium iron phosphate batteries in overcharge thermal runaway characteristics, but the trend of gas production type and gas concentration is basically the same. Reaction phenomena of lithium iron phosphate batteries in different stages of thermal runaway are interrelated with the change of gas concentration. H₂, CO and CO₂ can be used as the first-level warning, HCl and HF can be used as the second-level warning. By monitoring the change of gas concentration, countermeasures can be taken in the early stage of overcharge to avoid further thermal runaway combustion of energy storage batteries. The results of this study can provide effective theoretical and experimental supports for early warning and fire prevention of lithium iron phosphate energy storage power plant.