For tunable diode laser absorption spectroscopy, we propose a method of extracting the second harmonic (2f) signal by cutting off the absorption signal for segmental fast Fourier transformation, and the proposed method is compared with the harmonic signal processing method of traditional orthogonal lock-in amplifier. C2H2 absorption signal is generated based on MATLAB software, and functions of orthogonal lock-in amplifier and segmental fast Fourier transformation are verified, respectively. A sensing signal processing platform is realized for orthogonal lock-in amplifier and segmental fast Fourier transformation based on LabVIEW software. A near-infrared C2H2 sensor system is established by combining the 1.533 μm distributed feedback near-infrared laser and a developed Herriot gas cell. The sensor calibration and Allan standard deviation measurements are carried out with the prepared C2H2 gas samples. The results show that both methods possess a good goodness of linear fitting and reveal a similar stability.