Abstract: Cleat is extremely developed in coal seam, which leads to strong heterogeneity. During drilling in coal seam, borehole wall is easy to collapse and bury drilling tool.It is of great importance to prepare reasonable drilling fluid for keeping the stability of borehole wall. At present, the classical continuum theoretic model maily is used to forecast the density window of safe drilling fluid, resulting in poor actual effect in site. As a matter of fact, there are not only developed cleats in coal seam, but also a large amount of CH₄ occuring as adsorbed gas, weakening the mechanical strength of coal and making the borehole wall more unstable. Thus in view of the particularity of coal seam as a non-continuum medium, the authors used the discrete element model constructed on the basis of the continuum mechanics to analyze the borehole stability of coal seam in the process of drilling, and determined the best anti-collapse drilling fluid in combination of the analysis method of continuum medium strenghth and in consideration of the influence of gas adsorption on the weakening of the mechanical strenghth of coal and rock.Finally the case calculation using the above theory was conducted according to the coal seam data and in-site drilling fluid data from Ningwu basin in Shanxi province. Results showed that the density of anti-collapse drilling fluid must be greater than 1.009 g/cm³ and plugging agent needs to be added in drilling fluid to improve its sealing capacity.