Abstract: Tunable diode laser absorption spectroscopy is widely used in many applications，with difficulty of abstracting baseline，it is hard to be used in high-pressure and high-disturbing conditions. It's indicated in theory of wavelength-modulation spectroscopy that the first and second order harmonic signals contain tow common terms: incident intensity and gain of detector. By normalizing 2f signal with 1f signal (2f/1f), dependency of 2f/1f signal on incident intensity was eliminated, absolute strength of a spectra can be obtained subsequently. With measured laser parameter and known spectral parameters, theoretical 2f/1f signal was simulated by numerical calculation. Calibration-free wavelength-modulation spectroscopy was built based on frequency demultiplexing method, by numeric lock-in code. When a temperature stabilized cell was set at 600 K, 700 K, 800 K, precision of 2% is achieved compared with thermocouple measurement, which indicates that the method is much more reliable and adaptable.