Abstract: In order to solve the wellbore stability problems of coalbed methane（CBM）wells, a formula for calculating the collapse pressure for the horizontal well was established, using the mechanical methods to analyze the stress distribution of sidewell and separately adapting the Mohr-Coulomb strength criteria and the Drucker-Prager strength criteria to the direction well stability model. The formula is used to forecast the collapse pressure for the pinnate horizontal well in Qinshui 3# CBM fields. The results show that the wellhole is stable when it is in the direction of the maximal horizontal stress, in which case is that the collapse pressure of minus value; the wellhole is unstable when it is in the direction of the minimal horizontal stress, in which case is that the collapse pressure more than or a little less than the formation pressure,meaning it may cause coal dust; the collapse pressure is between the two when the wellhole is in the direction between the directions of the maximal horizontal stress and the minimal horizontal stress.