Abstract: Aiming at the problems of low hole-forming rate and shallow hole depth of gas extraction holes in broken soft coal seam, the mechanics model of borehole instability was established，and the mechanical mechanism of borehole instability was analyzed. The influence factors of the failure of borehole wall were expounded from the aspects of the coal structure, the ground stress, the gas pressure and the drilling process, and the damage mode of the borehole instability was pointed out. A numerical analysis model of borehole stability was established, the Hoek-Brown criterion was used to estimate the mechanical parameters of the broken soft coal seam. The stress field and displacement field around the borehole wall were calculated and analyzed. The results show that the bearing capacity of borehole wall in broken soft coal seam is poor. The failure mode of the borehole wall is shear failure, and the high stress increasing area is easily formed at the bottom of borehole to cause blowout easily; the broken coal zone moves toward the free surface firstly, the maximum displacement of the upper coal is 4.3 mm in the crushing zone, the maximum displacement of the borehole bottom is 3.1 mm.