Abstract: With the increase of new energy vehicles, the requirements for lithium-ion batteries expands rapidly.As the heart of electric vehicles, the safety of lithium batteries cannot be ignored.According to statistics, 80% of the failures of new energy vehicles caused by batteries.With the increasing popularity of new energy vehicles, their safety problems attract more and more attention.Regarding the reasons of thermal runaway accidents of lithium batteries, this paper will analyze the mechanism of thermal runaway of lithium batteries (including external causes and internal causes) and coping strategies, and put forward corresponding countermeasures for the safety improvement of lithium ion batteries, providing theoretical basis for the design of high-safety lithium ion batteries.