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To prevent economic losses caused by borehole breakout during coalbed methane extraction, it is essential to understand the mechanisms of borehole deformation and the distribution patterns of stress-induced fractures. However, previous research efforts have failed to address these aspects adequately. This article investigates the geometry nature of borehole breakout in pre-loaded coal samples under various factors using the discrete element method (DEM). The results revealed significant distinctions in borehole breakout behaviour between coal and other geological materials. In the case of coal, the geometry of breakout is particularly pronounced during the initial stages of failure. However, as stress differentials intensify, the geometry undergoes relatively minimal alterations compared to other rock formations, suggesting a relatively weak sensitivity. The geometry of the borehole breakout in the coal seam widened and deepened as the stress difference increased and the coal elastic modulus decreased. The breakout volume first increased significantly and remained stable as the stress difference increased. A critical threshold line is proposed to represent the macroscopic failure of coal rock caused by drilling, considering the combined effects of elastic modulus and stress differential. The region below the threshold line represents the trend of macroscopic failure after drilling instability, while the region above the threshold line indicates a stable trend. The findings of this study have practical implications for engineering applications. Highlights: Various factors (stress difference, borehole size, coal elastic modulus) influence the geometry nature of borehole breakout in coal. Breakout depth and breakout width in simulated coal rock show a linear positive correlation with stress differences. Borehole deformation in coal seams is apparent once it occurs but shows lower sensitivity to the in-situ stress difference compared to sandstone and limestone,. A decisive criterion for determining whether borehole breakout leads to macroscopic fracture in coal rock is suggested by considering the joint impact of elastic modulus and stress difference. [ABSTRACT FROM AUTHOR]