An absorptivity function reproduction method based on even harmonics belongs to the technical field of tunable laser diode absorption spectrum. Based on the wavelength modulation theory, a high frequency modulated laser transmittance function is expanded in time domain by Taylor series at the center frequency of a spectrum line. A harmonic expression obtained based thereon is composed of the lasertransmittance and the derivative thereof and the frequency modulation amplitude. An absorptivity function reproduction method based on even harmonics is established by analyzing the characteristics of the relationship between harmonics, and the measurement accuracy of the method increases with the increase of the order of harmonics adopted. The method establishes the relationship between the wavelength modulation spectrum and the absorptivity function, and solves the problem that the gas parameters need to be measured by calibration experiment because the absolute absorptivity cannot be measured effectively in the traditional wavelength modulation spectrum application.