Abstract: Based on the analysis of rock components and physicochemical properties for coal bed methane reservoir, this paper studied the influence of different physical and chemical properties on borehole stability of coal bed by measuring the coal rock mechanical properties when it contacted with the drilling fluid. The results indicated that clay expansion easily happened when drilling fluid invading into the coal beds, so the coal bed methane reservoir potentially occurred borehole instability induced by water sensitivity; we adopted water drilling in strong rock strength and non-water sensitivity coal bed, while clay inhibitor was added into drilling fluid in strong water sensitive reservoir to control the drill fluid invasion., combining with the under-balanced drilling.