With the continuous improvement of electrochemical energy storage technology in power systems, the safety of electrochemical energy storage technology has received more and more attention. It takes the lithium iron phosphate battery module (8.8 kW·h, 25.6 V, 344 A·h) for energy storage as the research object, and conducts three constant current overcharge tests with different magnifications (0.4C, 0.5C, 1C) to study its advantages. The thermal runaway characteristics of overcharge under different charging rate conditions are supplemented by starccm+ software for thermal field simulation calculations. The experimental results show that the battery module catches fire under 0.5C (172 A) and 1C (344 A), and the fire-starting time decreases as the charging rate increased. Charging rate has a great influence on the thermal runaway behavior of lithium iron phosphate battery modules. With the increase of charging rate, the thermal runaway maximum temperature and peak voltage increase, and the overcharge process time decreases with the increase of charging rate. Under different charging rates, the charge voltage when the first battery safety valve opens are 1.7 times of the rated voltage, which is further regarded as a warning parameter for battery thermal runaway. A reference for the safety improvement of lithium iron phosphate batteries for energy storage and the battery management system (BMS) for the safety management of overcharge faults is provided.