The geometric shape of hydraulic fracturing fractures is the main factor to evaluate the fracturing effect. The coal seam is affected by cutting, bedding and natural fractures, and the heterogeneity is strong. The propagation law of hydraulic fracturing fractures in coal seam under the combined influence of vertical principal stress, maximum horizontal principal stress and minimum horizontal principal stress has not been systematically understood. In this paper, RFPA-3D numerical simulation software is used to study the propagation process and extension law of hydraulic fracturing three-dimensional cracks in hard coal seam and soft coal seam with non-uniform distribution under the condition of constant vertical principal stress and different horizontal principal stress difference. It is found that when the horizontal principal stress difference of hard coal seam is close to that of soft coal seam, there is no dominant direction for the extension of hydraulic fracturing cracks in coal seam, and mainly extends along the horizontal principal stress, and then gradually closes to the direction of the maximum a mesh fracture is formed. When the horizontal principal stress difference is large, the hydraulic pressure crack horizontal principal stress. In the initial stage of hydraulic fracturing in soft coal seam, the coal seam near the injection section is squeezed around the wellbore, and the coal seam is squeezed to form a " big belly " area. When the coal seam is squeezed to a certain extent, the coal seam begins to crack and crack propagation. When the horizontal principal stress difference is close, after the compaction of the coal seam in the injection section, the crack initiates to form a mesh seam ; when the horizontal principal stress difference is large, after the coal seam in the injection section is compacted, two dominant fractures are formed along the direction of the maximum horizontal principal stress and the direction of the angle of 45°with the maximum horizontal principal stress. The research results were applied in Y-01 coalbed methane horizontal well group in Luling well field, Anhui Province, and the daily gas production exceeded 10 000 m3.