Thermal runaway and fire extinguishing are both urgent problems in large-scale applications of lithium iron phosphate battery (LFP) modules. In this paper, the thermal runaway characteristic of a single LFP module and a cluster of LFP modules are studied under constant current overcharge mode. Water mist is used as a fire extinguishing agent to study its fire extinguishing effect on single and cluster LFP modules. The experimental results show that the temperatures of the modules in the cluster rise sharply after combustion, with a temperature rise rate of 42.74℃/s in the first 18 s. The peak temperature is nearly 1 000℃, which is significantly higher than that of the peak temperature of the single module (600℃). After 100 seconds of continuous spraying of water mist, the temperatures of all the modules drop rapidly, and the fires are completely extinguished without reignition, thus proving that the fire extinguishing effects by using water mist are satisfactory. Above results provide an effective theoretical and experimental support for the safety and fire extinguishing of the LFP-based energy storage power station.