A tunable diode laser absorption spectroscopy technique using quantum cascade lasers and a movable open optical path system are proposed to realize on-line detection of SF6 gas in the leakage region. The optical path system is located around the interval of high-voltage combined appliances, and through the non-contact combination with the sensing system. The emission frequency range of the mid-infrared laser with injection current of 680 mA can cover the selected wavelength of SF6 gas at work temperature of 10℃. The precise background absorption of the second harmonic absorption is obtained by extracting and separating the internal environment of the large area laboratory module for several times. The leaked SF6 gas after diffusion equilibrium is measured by using the tunable laser sensor system with 10.55 μm. After subtracting the background absorption, the second harmonic absorption spectrum of the leaked SF6 gas was obtained. Research results show that under the open light path of 14.3 m, gas chromatography-thermal conductivity detector traceability of calibration data with tunable diode laser absorption spectroscopy technology testing consistency is well, the maximum deviation is 2.26%. The second harmonic signal amplitude and largest measuring concentration have a high fitting degree with fitting coefficient of 0.998, and SF6 volume concentration of the detection limit is 1.8×10－6. The study can provide a new detection method for SF6 gas leakage, so as to realize real-time online monitoring of SF6 gas under large area environment of high voltage combined appliances.