Abstract: The permeability of our country’s coalbed methane reservoir is low, so, even slight scale is most probably lead to reduce the permeability.The scaling trend of coalbed methane reservoirs in Qinshui zhengzhuang, Shanxi is predicted by the saturation index method based on the analysis of characteristics of coalbed methane reservoir when drilling fluid invade. Then the amount of scale is measured by static experiments under different mixing ratios of drilling fluid and formation water as well as different pH. The damage of formation by drilling fluid invasion is studied by core flow experiments. Theresult shows that calcium carbonate scale will develop after drilling fluid invasion, the amount of scale will increased with a higher pH value; the more serious the invasion of drilling fluid is, the greater the damage of coal reservoir will be.