To investigate the impacts of ambient pressure on thermal runaway and fire behaviors of lithium-ion battery (LIB), experimental measurement and theoretical analysis with serial conditions are conducted at two altitudes. The well-designed experimental equipment and operating conditions have enabled the accurate evaluation of ambient pressure effects. Results show that the first abrupt temperature change in Hefei (ambient pressure 100.8 kPa) is higher than that in Lhasa (64.3 kPa). The difference in ambient pressure at two altitudes leads to different relief valve crack temperature and time. The average burning rate in Hefei is larger than that in Lhasa, and the estimated pressure effect factor is quite different for detailed pack conditions and varies within the range of 0.083-1.39. The ambient pressure has a greater effect on the heat release rate and total heat release than the mass loss, and the effective combustion heat under the low pressure is lower than that in normal condition. This work can provide more comprehensive and useful data for the safety management of LIBs at low pressure environments.