Abstract: In order to solve the common technical problems of coal water swelling, swapblock in coal gas drilling, combined with the characteristics of negative charge on the surface of coal in water system, organic MMH-1 and inorganic MMH-2 were used as the main agent separately and a series of experiments were carried out wit h viscosifier and fluid loss additive. According to test and optimize drilling fluid rheological properties, filtration loss,p H and other basic performance indicators, we propo sed two kinds of MMH drilling fluid formulas for coalbed methane drilling. The experimental results showed that MMH drilling fluids had proper viscosity and shearing for ce, cutting carrying capacity, low fluid loss and good lubricating performance. Compared with 5% KCI solution, the swelling capacitiesof MMH drilling fluids were only one quarter which could effectively inhibit the expansion of coal rock and improve wellbore stability of coalbed methane wells.