The overcharge of the lithium iron phosphate (LiFePO4) batteries usually leads to the sharp capacity fading and safety issues, especially under low temperature environment. Thus, investigating their root cause originated from the electrode materials is critical for the safety performance optimization and market promotion of the LiFePO4 batteries. In this work, the electrochemical/thermal behaviors of 18650 LiFePO4 batteries are investigated after overcharge under room and low temperature of 25 degrees C and -20 degrees C, respectively. The results demonstrate a decreased electrochemical performance and faster heating rate of the overcharged battery, particularly under harsh working environments such as high discharge rate and low temperature. Coupling with the analyses of the internal resistance, the crystal structure, and microstructure of the electrodes, the root cause is attributed to the damage of the crystal structure and microstructure, which reduce the electron/Li+ migrating capability and electrolyte diffusion/transfer efficiency.