The development process of long air gap discharge is accompanied by the phenomenon of dielectric thermal ionization. Accurate temperature field acquisition is conducive to the in-depth analysis of gap discharge characteristics. The background oriented schlieren（BOS） breaks through the limitation of lens size in traditional schlieren technology and can obtain the temperature distribution characteristics of large-scale discharge channels. In this paper

the background oriented schlieren measurement system is built

the background spot atlas in the development of discharge is collected

and the ray offset through the discharge channel is analyzed by PIV particle image velocimetry technology; the refractive index corresponding to the ray offset is calculated based on Abel inverse transformation

and then the density field and temperature field are reconstructed quantitatively according to the thermodynamic equation and ideal gas state equation. The temperature calibration of alcohol lamp flame is carried out

and the analysis results are basically consistent with the measured results of thermocouple. A 1 m rod plate air gap is taken as the research object

through the background schlieren measurement system

the development process of gap positive discharge is observed

and the temporal and spatial variation law of discharge channel temperature is analyzed. The results show that the temperature distribution of the cross section in the middle of the gap is about 1 200 K

and the temperature field is axisymmetric. The closer the channel is to the center

the greater the temperature gradient will be

and the temperature of the whole channel decreases gradually from the pole.