Abstract: According to the on-site data, this paper identified the formation reason, production rule and prevention method of coal dust in horizontal well. Research results show that the coal dust may produce because of the variation of stress on the borehole, coal rock material components and composition, drill tool grinding, coal seam bending section of well and working system change such as to make the liquid drop rate, the content of clay minerals will greatly influence the output of coal dust as well. During the whole production period, the diameter of coal dust may vary between a few micrometers to several hundred micrometers. In the very beginning, stress changes on the well bore and the grinding effects are the main reasons for coal dust production, so then it yields relatively large particles. But in the later period, it is mainly influenced by the variation of production rate and the amount of clay minerals, which result in minor coal dust. In order to prevent coal dust production, it is crucial to pay attention to the following aspacts:first, clean-up the well, especially the horizontal section thoroughly, second, optimize the length, diameter and the size of the slot of the screen pipe, third, optimization of production equipments, finally, establish a rational production rate.