Abstract Coal seam fracturing and water injection are important techniques for increasing permeability in joint coal and gas mining operations. The mechanism of fracture development during water injection fracturing has always been an important yet fully understood scientific issue. Scholars have conducted many studies on macro-scale fracturing laws and micro-meso-scale structure characterization, but few on the development laws of fracture networks under the macro-meso scale. In this study, scanning electron microscope (SEM) technology, image digital processing, simplified model processing, and fractal algorithm verification methods were applied to obtain an expanded macro-scale model. A global embedding cohesive model was adopted to study the fracture development law of water-injected coal samples under different stress differences. The fracture development laws at the meso and macro scales were determined, two modes of fracture development at the meso scale were found, and fracture development was divided into three stages. All the natural fractures in the development coverage of fractures were included in the development process. The effect of natural fractures on the development direction of the macro-scale fracture and the rule that the macro-scale development direction of fractures is parallel to the meso-scale natural fracture direction were observed. Pore pressure was closely related to the development process of the fracture. The increase in pore pressure at the tip of the fracture signified the start of fracture division and density increase. Finally, the relationship equations between parameters, such as water injection pressure, macro-scale development range, and initial fracture width, were also determined.