Abstract: In the process of drilling in broken coal seam, the borehole instability-related problem is serious. Traditional mechanical model is difficult to evaluate the failure mechunism of borehole. In this paper, based on the characteristics of broken coal seam, a case study was conducted in 3# coal seam of the Qingduan block, Qinshui basin, the coal and the rock was defined as two parts:matrix and fissure. Monte Carlo method was used to generate the random fissure network. The effect of drilling methods on wellbore stability was numerically simulated by the universal discrete element code (UDEC). The results show that micro underbalanced and micro balanced drilling can be adopted in coal drilling. Tensile failure occurs at low wellbore pressure; coal body is gradually sheared under big wellbore pressure, the best stable wellbore pressure interval value is 6~7 MPa in this case.