Abstract: Under the dual constraints of industrial energy consumption and emission control, accurate assessment of combustion efficiency is of paramount importance. Oxygen, as a representative oxidizing agent involved in combustion reactions, serves as a key indicator for evaluating combustion conditions by means of its residual concentration in high-temperature flue gas. For industrial high-temperature and high-humidity flue gas environments (water vapor volume fraction of 10%-50%), conventional electrochemical and gas chromatography methods exhibit limitations in accuracy, reliability, and temporal response when measuring residual oxygen. To address these challenges, this study developed a laser absorption spectroscopy-based sensor capable of quantitatively measuring residual oxygen in high-temperature and high-humidity flue gas. The sensor employs a 760 nm laser source and an 18 m high-temperature multipass gas cell, and it combines these with the wavelength modulation spectroscopy (WMS) technique while optimizing modulation parameters to effectively suppress high-humidity background interference. Experimental results demonstrate that the system offers a wide dynamic detection range (0.013% to 5.000%) within 1 s and maintains high linearity (linearity of fit R²\geqslant 0.999) under high-humidity and high-temperature (473 K) conditions. Allan variance analysis indicates a minimum detection limit of 17×10⁻⁶ at an integration time of 183 s in high-temperature environments. Continuous measurement tests further verify the sensor’s rapid response to oxygen concentration variations and its excellent stability in high-humidity flue gas, providing a reliable solution for high-precision, wide-range, real-time monitoring of residual oxygen concentrations in high-temperature and high-humidity conditions.