Abstract: The effective porosity of coal body is closely related to coal seam enhanced permeability effect. According to this feature, it takes difficult water injection of poorer coal seam permeability as the research object, and studies the impact of pulsation water hammer enhanced permeability technology on the effective porosity of coal under different water injection pressure. The test shows that when it uses the conventional static pressure water injection, effective porosity value of coal is low, with the increase of injection pressure, its growth rate is smaller. After using pulsation water hammer enhanced permeability technology, coal has high effective porosity in the lower water injection pressure,enhanced permeability effect is obvious, and in the whole process of water injection, coal can keep high effective porosity.