Lithium ion battery and its safety are taken more consideration with fossil energy consuming and the reduction requirement of CO2 emission. The safety problem of lithium ion battery is mainly contributed by thermal runaway caused fire and explosion. This paper reviews the lithium ion battery hazards, thermal runaway theory, basic reactions, thermal models, simulations and experimental works firstly. The general theory is proposed and detailed reactions are summarized, which include solid electrolyte interface decomposition, negative active material and electrolyte reaction, positive active material and electrolyte reaction, electrolyte decomposition, negative active material and binder reaction, and so on. The thermal models or electrochemical–thermal models include one, two and three dimensional models, which can be simulated by finite element method and finite volume method. And then the related prevention techniques are simply summarized and discussed on the inherent safety methods and safety device methods. Some perspectives and outlooks on safety enhancement for lithium ion battery are proposed for the future development.