The coal-fired boiler combustion process's economic, safety, and environmental performance holds great significance when constructing smart power plants. In coal-fired boiler combustion, H₂S and CO are the two main high-temperature corrosive gases. They not only corrode the boiler near the wall surface but also pose severe harm to the atmospheric environment through their exhaust gases. Based on the near-infrared tunable diode laser absorption spectroscopy technology, combined with wavelength modulation spectroscopy and frequency division multiplexing technology, an unstaffed online real-time monitoring instrument for H₂S and CO gas concentrations in the main combustion zone of coal-fired boilers was developed. Gas absorption spectroscopy in the 6335~6341 cm^-1 range was simulated, and two near-infrared lasers near 1.5 μm were selected as the laser source. A high-temperature resistant and corrosion-resistant Herriott-type multi-pass cell was developed to attain an effective optical path length of 15 m for the interaction between laser and gas. Hardware circuits and corresponding firmware programs were developed to attain secondary demodulation of the absorption spectroscopy signals of H₂S and CO and concentration inversion. The linearity and Allan variance experiments showed linear fitting correlation coefficients of 0.9998 and 0.9995. At 73 s and 53 s integration times, the minimum detection limits for H₂S and CO were 0.2×10^-6 mol/mol and 0.344×10^-6 mol/mol, respectively. Finally, the developed instrument was applied in the combustion atmosphere of the main combustion zone of a 300 MW tangential four-corner coal-fired boiler, and synchronous measurements of H₂S and CO near the water-cooled wall were conducted. The results indicated a positive correlation between the concentrations of H₂S and CO in the boiler, with anaerobic combustion leading to an increase in the content of these gases and causing corrosion to the water-cooled wall.