Most of the natural gas mines in southwestern China are high-sulfur gas mines. Among high-sulfur gas, H₂S presents a dual threat due to its high toxicity and propensity for corroding pipelines and instruments, which poses substantial safety hazards to gathering and transportation systems of natural gas. Therefore, rapid and precise monitoring of H₂S level in natural gas is of great significance for improving the safety and efficiency of sulfur-laden natural gas production. In response to this demand, a portable H₂S online detector based on tunable diode laser absorption spectroscopy (TDLAS) technology was developed in this work. The main body of the detector was a high-temperature long-path absorption cell with a base length of 15 cm, and it adopted a seven-ring spot scheme with an effective optical path of 30 m. In combination with a pulse width modulation controller and mass flowmeter controllers, a digital proportional controller was used to achieve constant pressure and temperature in the absorption cell. A near-infrared laser with a central wavelength of 1578 nm was used as the laser source of the detector, and based on complex programmable logic device and analog phase-locking technology, the second harmonic (2f) modulation and demodulation of H₂S spectral signal were achieved. Furthermore, the hardware system and software program for the control end of the portable detector were developed, which enables real-time display, storage, and transmission of H₂S concentration. The detector was calibrated using H₂S standard gases, and the results showed that the linear correlation coefficient (R²) between the detected 2f spectral signal of H₂S and H₂S concentration is high to 0.99786. Finally, a field test of the developed instrument was conducted at a natural gas pigging station in southwestern China, and the test results showed that the H₂S concentration in the natural gas pipeline was high, with a peak value of 1.16 mg/m³, and the pipeline corrosion was severe on site, which preliminarily demonstrated the applicability of the developed portable H₂S online detector.