Abstract: At present, lithium-ion batteries are widely used in electrochemical energy storage systems. However, due to the differences between batteries, single batteries often have over discharge phenomenon, which brings safety risks to the application of battery modules and energy storage systems. Therefore, the detection and analysis of lithium-ion batteries in over discharge state are of great significance for their safety applications. Cycle tests of normal cycle and different degrees of over discharge are designed. The relaxation time distribution method, impedance difference analysis method and capacity increment method are used to analyze the impedance characteristics of lithium-ion batteries in the whole life cycle of over discharge state. The results show that over-discharge can accelerate battery aging and increase the temperature rise during charging and discharging. Compared with the normal cycle, the ohm internal resistance and charge transfer resistance increase in the over discharge cycle, the internal resistance of SEI film decreases, and the charge transfer resistance has no significant change. The solid-phase diffusion resistance of lithium-ion decreases first and then increases with the increase of over discharge degree. The results provide a theoretical basis for the study of the internal characteristics of lithium-ion battery over discharge and the detection of over discharge.