Abstract: Thermal runaway of lithium-ion batteries is the fundamental cause of safety accidents such as fire or explosion in energy storage power stations. Therefore, studying the development law and intrinsic characteristics of thermal runaway of lithium-ion batteries is important for the safety monitoring and fault warning of electrochemical energy storage power stations. In this paper, a three-dimensional electrochemical-thermal coupled LiFePO₄ battery overcharge thermal runaway simulation model is established. Firstly, the amount of lithium plating on the overcharged negative electrode is quantified by the lithium plating kinetic equation, and secondly, the SEI film growth kinetic equation is introduced to reflect the lithium plating and electrolyte. The reaction rate is used to quantify the heat generated by the reaction between the negative electrode lithium plating and the electrolyte, and other side reaction heat generation equations are introduced to jointly study the temperature change of the lithium iron phosphate battery during the early overcharge and the runaway temperature and the heat generation of each side reaction. The changes in the amount of lithium plating on the negative electrode surface in the early stage of thermal runaway of lithium iron phosphate batteries under different charging rates (1C, 2C, 3C) and different ambient temperatures (20 ℃, 30 ℃, 40 ℃), the temperature curve of thermal runaway, and the change characteristics of the heat generated by the reaction are analyzed, and the development process of the thermal runaway temperature of the lithium iron phosphate battery and the law of the side reaction heat generation are analyzed. The results show that the reaction between the negative electrode lithium plating and the electrolyte is the initial side reaction in the thermal runaway process of overcharge, which promotes the opening of other side reactions in the early stage of thermal runaway of the battery, which becomes the beginning of thermal runaway of overcharge. This study can provide a theoretical reference for the early process of overcharge thermal runaway of LiFePO₄ batteries.