Abstract: Coal seam water injection is a widely used fully mechanized mining method for dust suppression. The pore structure and stress state change when water pressure acts on the coal during water injection, but the pore structure is the primary factor affecting the effect of coal seam water injection. In order to better reveal the evolution law of coal pore structure and stress sensitivity during coal seam water injection, low field nuclear magnetic resonance monitoring and in-situ coal seam water injection technology are used in this paper to evaluate the stress sensitivity of coal multi-scale hole fractures of drilled and undrilled coal cores respectively under the condition of dynamic load coal seam water injection. The change law of coal pore diameter and compression coefficient with coal seam water injection is quantitatively characterized, and its dynamic mechanism is described in combination with effective stress. The results show that the pore compression coefficient of different scale pores is not a constant, but changes with the change of stress, and the degree of change is related to the mode and time of stress action. Confining pressure and water pressure have opposite effects on pore compressibility at each scale, mainly due to the effect of effective stress on pore structure.