This paper mainly presents the experimental research on the optical properties of high temperature, low density air using a 800 mm diameter shock tube and a radiometer. The experimental temperature ranges over 4380~7500K; the density ranges over 1.41×10-2~1.00×10-1 ρ0 (ρ0 is the standard density at sea level). The spectral wavelengths are 430.0, 540.0, 640.0, 720.0 and 1038.0 nm. The experimental results are compared with the theoretical and experimental results published by other countries and shown a good coincidence. The characteristics of flow field are given and the reliability of experimental results is analysed too.