To achieve the online identification and detection of trace ethylene gas during coal pyrolysis, we constructed a vinyl gas concentration detection system using tunable diode laser absorption spectroscopy (TDLAS). This system combines wavelength modulation with long optical path technology. We employed a distributed feedback (DFB) laser with a central wavelength of 1 620 nm, situated in the communication band, and a 15⁃meter long optical path cell for sample absorption. The SR830 was used for wavelength demodulation, enabling us to determine ethylene concentrations through second harmonic signal inversion. We used high⁃precision flow controllers to dilute ethylene with high⁃purity nitrogen gas, creating standard ethylene gas samples with concentrations ranging from 10×10^-6 to 90×10^-6, achieving a linear fitting correlation coefficient R² of 0.998 9. An Allan variance analysis experiment on a 20ppm ethylene sample over 4 000 s determined the minimum detection limit to be 121×10^-9. To examine the evolution of ethylene concentration in various gas environments during coal pyrolysis, we maintained a gas flow rate at 150 mL/min. We analyzed the online release process of identified ethylene gas in nitrogen, air, and synthetic air (22% oxygen, 78% nitrogen) environments. Our findings revealed that below 500 ℃, ethylene release in all three environments was low and consistent. Between 500 to 700 ℃, ethylene release in a nitrogen environment was notably higher than in the other two gases, with the release rate in air being the fastest, peaking at about 40ppm. Above 700 ℃, ethylene release in all environments began to decline. These results provide a critical scientific basis for further exploration into the mechanism of ethylene generation during coal pyrolysis, the refinement of coal pyrolysis techniques, the enhancement of coal utilization efficiency, and the advancement of environmental preservation.