The continuous monitoring of H2S gas concentration is a common problem in natural gas desulfurization process technology. Tunable diode laser absorption spectroscopy (TDLAS) is a preferred technology for continuous monitoring of gas in industrial sites, because of its high selectivity, high sensitivity, and fast response. We discuss the technical solutions of on-line monitoring of H2S in natural gas desulfurization process technology based on TDLAS, and study the security design of monitoring system in inflammable and explosive areas. We also design a weak photocurrent signal converting circuit and perform experiments on transmission characteristics of different distances. The signal-to-noise ratio (SNR) of laser absorption spectrum does not decrease after the 1 500-m transmission. The detection limit is 300 ppb. The system can be operated stably and reliably, and satisfies the need for continuous monitoring of the H2S in natural gas desulfurization process.