Thermogenesis mechanism in the aspect of structural level and thermal hazard to the lithium ion battery are systematically analyzed for Li(Ni x Co y Mn z )O2(NCM, x =1/3, 0.5, 0.6, 0.8). All the results confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, alternating current impedance and a C80 micro-calorimeter indicate that with the increase of nickel content, the structure of NCM materials tends to collapse causing heat and the valance of Ni element is high enough to oxidize the electrolyte accelerating the decomposition of the electrolyte, also some electrolyte decomposition products are speculated to cover on the surface of the cathode with larger inner resistance for the battery. The thermal tests display that the total exothermic heat of both NCM-electrolyte system and full-cell system increases with increasing nickel content, which testifies the risk of high-nickel NCM to the lithium ion battery.