Shale gas exploration and development have become a hot spot in the natural gas industry. Providing wellbore stability is one of the major engineering problems occurring in the process of drilling of horizontal shale gas wells. In this paper, based on circumferential stress analysis of horizontal wells and analysis of the geometrical relationship of bedding planes and the wellbore, a new horizontal wellbore stability model is introduced. The proposed model takes into account the effect of bedding planes and water content on the strength parameters of shale. Based on actual field data of a shale gas well, the proposed model is compared with the wellbore stability model for an intrinsic rock formation. The results show that the existence of bedding planes affects the stability of a wellbore, making it less stable. Meanwhile, the maximum and minimum values of collapse pressure density for different azimuths of wellbore increase linearly with increase in water content. Moreover, the results also show that factors of the wellbore azimuths and the bedding planes dip angle and dip direction have a significant impact on the wellbore stability of horizontal shale gas wells.