Flame soot is an import product as a result of incomplete combustion of hydrocarbon fuels, and has important effect on the generation of some pollutant. The optical properties of the soot are the base for the optical combustion diagnostics. Terahertz time-domain spectroscopy technique was used to study the optical properties of the soot within 0.2－1.6 THz and the frequencydomain spectra were obtained through Fourier transform. The complex refractive index of the soot was deduced by the fixed-point iteration. The comparison of the complex refractive index between terahertz spectra domain and thermal radiation spectra domain was conducted and two methods of extracting the parameters were also compared. The results indicated that the soot has strong absorption in the studied spectra and the difference in the refractive index between the two spectra domains was not obvious, but the absorptive index variation in the thermal radiation spectra domain was larger. The difference between the two parameter extraction methods was not significant. The deduced results can provide the optical data of soot for the application of terahertz time-domain spectroscopy technique to the optical combustion diagnostics, and extend the optical combustion diagnostics application area.