Based on wavelength modulation spectroscopy (WMS) theory, a measurement method for gas parameters in combustion flow is put forward using spectroscopy fitting. Taking advantage of fitting harmonic signals of spectral lines, the integral absorbance, Doppler width, and collisional width are measured, and then the gas temperature, pressure, and vapour concentration are measured. The numerical simulation is used to investigate the effect of the integral absorbance and collisional width on harmonic signals, and experiments are conducted in a sample cell. The results show that the sensitivity of the harmonic signal spectrum to integral absorbance is nearly 1. However, the sensitivity of the harmonic signal spectrum to collisional width firstly increases and then keeps invariant approximately with the increase of collisional width. The fitting method has a high measurement accuracy. Compared with predicted values, the maximum measurement errors of measured temperature, pressure, and vapour mole fraction are less than 4%, 6%, and 5.5%, respectively.