Lithium-ion batteries (LIBs) with relatively high energy, power density and eco-friendly characteristic are considered as a vital energy source in consumer market of portable electronics and transportation sector especially in electric vehicles (EVs). To meet the higher capacity requirements, the nickel-rich LIBs with higher capacity has been used as the commercial power batteries. However, the battery with higher energy density is more destructive, which could result in thermal runaway accidents and make the battery safety issues become more and more prominent. Thermal runaway of LIBs is one of the key scientific problems in safety issues. Until now, the inducement of thermal runaway process is complicated which perplex researchers and industry a lot. On the one hand, the internal mechanism about thermal runaway should be deeply studied. On the other hand, in-situ monitoring should be developed to supply the mechanism study and early warning. In this paper, the latest evolution about LIBs thermal runaway is highlighted. The internal mechanism of chain reactions during thermal runaway process is summarized. Furthermore, different in-situ monitoring technologies to investigate the thermal runaway process are discussed and analyzed including accelerating rate calorimetry (ARC), X-ray computed tomography (CT), in-situ scanning electron microscopes (SEM), in-situ transmission electron microscope (TEM), infrared real-time gas analyzers (ZRE) and in-situ X-ray powder diffraction (XRD) etc. With the in-situ monitoring technologies, it is expected to reveal the internal structure change of the battery during thermal runaway process and study the evolution of temperature, voltage, resistance and gas production during thermal runaway process.