With the rapid development of semiconductor devices, the research based on molecular vibration-rotation absorption spectroscopy has been deepened, significant progress has been made in detection methods represented by Tunable Diode Laser Absorption Spectroscopy (TDLAS), the application field has gradually expanded, and more than 1 000 instruments have been used in continuous emission monitoring and industrial process control and other fields, TDLAS-based gas detection instruments sold every year account for more than 5% of the total number of infrared gas sensing and detection instruments, and have achieved high-precision detection of parameters such as component concentration, temperature, and pressure in different fields. In this paper, in a complex application environment where the target absorption information in the instrument development field is completely covered, modulation technology is used to transfer the absorption information to the high-frequency part, and the absorption information is extracted through multi-channel background subtraction and harmonic signal normalization processing. Taking the measurement of trace H₂S in sulfur-containing natural gas as an example, when the CH₄ content in natural gas exceeds 90%, the absorption information of the target component H₂S is completely covered, and the absorption information is transferred to a frequency of 1 kHz. The peak value of the harmonic signal obtained is positively correlated with the H₂S concentration, with a linear parameter of 0.999 9, achieving effective extraction of weak absorption signals. The method verifies the effectiveness of extracting weak absorption signals. The effectiveness of extracting weak absorption signals is verified, which further expands the application field of optical gas sensing and provides technical support for instrument development.