Safety issue concerning “thermal runaway (TR) behavior” of lithium-ion battery (LIB) is one of the core concerns for users. We have studied TR behaviors at various ambient pressures. The thermal runaway onset time (t1) occured in advance at ambient pressure decreasing to 50 kPa from 90 kPa (90, 80, 70, 60, and 50 kPa). At 50 kPa, thermal runaway onset time of LIBs was 177 s earlier than that at 90 kPa. With the decreasing ambient pressure, several values declined, such as battery peak surface temperature, heat release rate (HRR), peak flue gas temperature, and total heat release (THR). Moreover, the peak concentrations of CxHy and CO increased as the ambient pressure decreased, whereas peak concentrations of CO2 and NO showed the opposite trend. Based on the previous studies of the thermal analysis kinetics model of LIBs, a pressure correction factor kp was introduced to establish a prediction model for thermal runaway temperature at low pressure conditions. Based on the model output, the error of thermal runaway onset time t1 could be controlled within ±2 s, and the error of thermal runaway peak temperature Tmax could be controlled within ±2 °C. Our results have been well consistent with the results of simulation, and have been beneficial to further reveal LIBs thermal runaway behavior under low ambient pressure.