Abstract: Flash point, one of the main indicators to evaluate the fire hazard, is an important physical property of the flammable liquids. Using a portable flash point measuring apparatus, a series of measurements of two mono compound fuels (n-decane and n-hexanol) and two mixture compound fuels (0# diesel and Jet A) at different pressures were carried out in an altitude chamber at 35, 45, 55, 65, 75, 85, 45 and 101 kPa. The experiment results show that the flash point is in a non-linear relationship with the pressure. And using the theoretical analyses from previous studies, it can be verified that analog low pressure is in good agreement with the real situation. Based on the classification codes in China, the fire hazard class of the flammable liquids at different pressures has been given. The results can provide a scientific basis for the production, storage, transport and use of fuel in the plateau environment, as well as fire safety in aviation.