Abstract: In order to accurately measure flame temperature by acoustic method, the relationship between sound velocity and temperature of air in flame burning region is studied experimentally. Firstly, the relationship between sound velocity and temperature of air in the non-flame burning environment is measured, and then on this basis, the experiments of the sound velocity of air in the flame burning region of different fuel combustion are carried out, and the relationship between sound velocity and temperature of air in the flame is obtained by measuring the flame temperature with thermocouple. The results show that at a fixed distance, the acoustic wave travel time is reduced in the high-temperature smoke environment than in the room-temperature air environment, but the flame environment makes the travel time longer. It means that in the non-flame region, the relationship between sound velocity and temperature of air is consistent with that of ideal gas. In the flame burning region, the relationship between sound velocity and temperature of air deviates from the equation of sound velocity and temperature of ideal gas, and the measured value of actual sound velocity of air is smaller than that calculated for ideal gas. For the same fuel flame, the sound velocity of air decreases with the increase of flame temperature.