Abstract: This paper studies the calculation method of gas-water relative permeability using fractal theory, through theoretical analysis and laboratory experiments. The results show that the higher the porosity and permeability of coal sample, the higher the largest mercury saturation, the larger the two phase flowing area, and the smaller the isosmotic water-gas saturation. It is feasible to calculate gas- water relative permeability curves of coal rock by using capillary pressure curve through fractal theory. The fractal dimension mainly affects the relative permeability of water phase. The larger the fractal dimension is, the more the water phase permeability curve deviates to the right section and the greater the saturation of the isosmotic water-gas saturation points. This suggests that the bigger the fractal dimension of coal reservoir, the greater the difficulty of water seepage. So the bottom hole flowing pressure should be controlled above the desorption pressure to avoid two phase flow which can expands the de-pressured and desorption area.