Abstract: For the situation that horizontal well coalbed methane exploitation in the Qinshui basin uses clear water as the drilling fluid, based on the analysis of rock components and physicochemical properties for coal bed methane reservoir by X-ray diffraction(XRD), Scanning Electron Microscope(SEM), coal rock swelling test and evaluation of sensitivity, combined with water scaling prediction, water analysis of grain size and excessive dynamic volumetric flow experiments to study the clear water drilling fluid, the results show that the factors caused damages of the coalbed methane reservoir include the bad compatibility with formation water which leads to scaled easily, the hydrate expansion of clay minerals in the coal bed with the low degree of mineralization, the weak self flocculated ability which is easy to cause the small solid particle blocking pollution, and the bad flow back ability which is against the coal-bed gas drainage exploitation.