Abstract: In view of the doubt about whether the harmful gas in the upper remote goaf area is discharged and influences the gas concentration at the corner of the working face under the influence of mining in the top-coal caving working face in extremely thick coal seam, a method of joint verification by borehole photography and tracer gas technique is proposed. The results show that the collapse height of strata in fully mechanized caving face of extra-thick coal seam rises in a step-like manner, and the collapse height is much higher than that of ordinary fully mechanized caving face; when the working face is mined to 100 m, the collapse height of the upper rock layer can reach 190 m. After the first pressure of the working face, the harmful gas in the upper goaf is discharged to the working face goaf through the collapse crack at a long distance, ultimately affecting the concentration of harmful gas in the upper working corner; drilling and photography combined with SF6 gas tracer technology can accurately determine the influence of harmful gas in long distance goaf on the working face, which provides technical support for the prevention and control of harmful gas on the working face.