Light-induced thermoelastic spectroscopy (LITES) technology is a novel trace gas detection technology that has developed rapidly in recent years. The technology employs miniature, cost-effective, and wavelength-insensitive quartz tuning fork as substitutes for high-cost, narrow-bandwidth photodetectors in the field of optoelectronic transduction. The target gas concentration is achieved by detecting the variation in optical intensity resulting from the interaction between laser radiation and the target gas. LITES technology has the advantages of high detection sensitivity, short response time, and independence of excitation wavelength. In this paper, the research of trace gas detection system based on LITES technology is carried out with hydrogen sulfide (H₂S) gas in sewers as the measurement target. A near-infrared distributed feedback laser with an output wavelength of 1.582 μm is employed as the excitation light source. The wavelength modulation spectroscopy and second harmonic detection techniques are utilized for trace gas concentration measurements. The impact of laser wavelength modulation depth on the signal amplitude generated by the LITES system is analyzed, and then the effect of operating pressure on the performance of the LITES system is also studied in detail. In addition, to further improve the detection sensitivity of the device, a Herriott multipass cell with an effective optical path length of 14.5 m is assembled between the laser and the quartz tuning fork. The sensor reached a minimum detection limit of ~4.87×10^-7 of H₂S with an integration time of 300 ms. By extending the integration time to 52 s, the minimum limit can be reduced to ~7.78×10^-8. Using optimized parameters, on-site measurements of H₂S in the sewer are conducted. The results indicate that the system is fully capable of meeting the application requirements in the fields of sewer odor monitoring and analysis.