Integrated absorbance (IA) can be used to infer gas temperature and concentration directly, in this paper, we proposed a new method that uses the 1st harmonic to measure the IA under low absorption conditions (<10%). Subsequently, a large number of numerical simulations are used to validate the reliability and accuracy of this method, and several absorption lines of CO₂ and H₂O molecules near 6981 cm⁻¹ are selected to determine the IA and species concentration in experiments. Calculation and experiment results show that the proposed method can accurately measure IA in actual measurements.