Abstract: Parameters of the high-temperature gas in the high-enthalpy flow field arean important basis for analyzing the thermochemical characteristics of the high-enthalpy flow and its interaction mechanism with exothermic materials. However, due to the high temperature of the flow field and severe chemical reaction, the measurement signals of these parameters are difficult to calibrate, which makes quantitative measurement difficult all the time. Wavelength Modulation Spectroscopy (WMS) is a kind of Tunable Diode Laser Absorption Spectroscopy (TDLAS), which has stronger anti-interference ability and is more suitable for complex environment application compared with the direct absorption method. A calibration-freed data processing method of WMS has been realized by fitting the measured harmonic signal with the theoretical harmonic signal, and applied in the diagnosis of the high frequency induction plasma flow and thearc-heated flow. The results show that the temperature and electronic density in the plasma flow resolved by the WMS are very close to those measured by the direct TDLAS, and the relative error of the specific enthalpy in the arc-heated flow between the WMS and the energy balance method is about 10%, which indicates that the free-calibrated WMS is reliable and can be used as an effective tool in the quantitative measurement of the high-enthalpy flow.