In mass flow measurement system, harmonic component of wavelength modulation spectroscopy signal, especially the second harmonic component is commonly used as test object, it is used for the inversion of gas concentration, velocity and other information. Based on the principle of wavelength modulation, using oxygen (O2) molecules in the vicinity of 764.28 nm absorption lines, the full width at half maximum (FWHM) of common line profiles is analyzed, and the range of application of the Lorentzian line profiles at different temperature and pressure is obtained; absorption spectral lines of O2 molecules are studied under different conditions of temperature, pressure, and the second harmonic peak change relation between them is studied as well; the error of the second harmonic peak inversion is analyzed under different temperatures and pressures, and error correction methods are put forward; in the experiment，when the optimal modulation depth is impossible to immediately adjust with the temperature and pressure changes, the optimal modulation coefficient application range under different temperatures and pressures is analyzed, and the error correction method is given.