Abstract: In order to study the pore-forming process of the rotary jet and the particle size distribution of the coal cinder after breaking coal. According to the theory of maximum tensile strain and the theory of maximum shear stress, the failure criterion of coal under shear stress and tensile stress is obtained. Based on the strength of 3.65 MPa, elastic modulus of 185 MPa, size of 1 m×0.8 m×0.8 m coal similar material, the coal breaking experiment under different pump pressures was carried out; the coal cinder from broken coal was collected and the particle size analysis experiment was carried out. The results show that the external drilling jet breaks the coal drilling speed and the pore diameter increases with the increase of the pump pressure. In the pore-forming process, the forward jet breaks the coal, the backward jet reams, and the failure of coal is mainly caused by tensile stress and shear stress. The coal cinder particle size distribution is from 84 μm to 1 061 μm. The larger the pump pressure, the smaller the particle size of the coal cinder.