The 26650 ternary LiNiCoMnO2 (NCM) lithium-ion battery has been widely applied in the development of electric vehicles due to its advantages of premium electrochemical performance and low cost. To investigate the impacts of ambient temperature and depth of discharge (DOD) on overdischarge behaviors of 26650 NCM lithium-ion battery, this paper has performed 100%, 110%, 120% and 130% DOD discharge at normal, low and high ambient temperature, and has carried out impedance analysis before and after the test. The overdischarge results indicate that at low ambient temperature, there is a delay in the occurrence of internal short circuit; during 110% ~ 130% DOD discharge, there may be severe voltage oscillations, while the interval for the battery to reach its maximum temperature and the overall temperature rise rate during overdischarge increase. At high ambient temperature, the occurrence of internal short circuit is delayed while the time required for the battery to reach its maximum temperature is longer. The impedance test results show that the variation in internal resistance significantly decreases after overdischarge at low ambient temperature. After 110%，120% and 130% DOD overdischarge at high ambient temperature, the variation rate of battery internal resistance is greater than that at normal temperature. This study reveals the failure mechanism of 26650 NCM lithium-ion battery overdischarged at different ambient temperatures and DODs, which will provide vital support for the setting of early warning parameters for overdischarge fault and the preventive methods for battery thermal runaway caused by overdischarge.