Abstract: The macroscopic and microscopic characteristics of water absorption and expansion deformation of coal samples were studied by thermodynamic equilibrium water method, low temperature nitrogen adsorption experiment, mercury intrusion experiment and scanning electron microscope (SEM).A similar simulation experiment method of hydraulic fracturing of borehole through underground coal seam was proposed, and the influence of water sensitivity damage on coal seam permeability was studied.The results show that the swelling rate of saturated water treated coal samples is most significantly affected by the content of water sensitivity expansive clay; after water saturation treatment, the intervention of water reduces the full-scale pore volume of coal, thus reducing the gas diffusion and seepage capacity of coal samples; within the effective range of hydraulic fracturing, the permeability coefficient of coal samples increases, and fracturing antireflection is dominant. But in the distal or terminal position, the permeability coefficient decreases, and water sensitivity damage is dominant; the higher the content of water sensitive clay in coal, the more serious the damage of water sensitivity. The research results are of great significance for reducing the water sensitivity damage of coal seam hydraulic fracturing and improving the efficiency of coal seam gas drainage.