Abstract: To reveal the impact instability mechanism of irregularly wide coal pillars under the cumulative effect of coseismic stress triggered by high-energy microseismic events. Using the engineering context of non-equidistant coal pillars in the “horizontal segmentation-tilted layering” fully-mechanized caving mining of a ultra-thick coal seam in a mine in Gansu Province, we employed a stress inversion method based on joint seismic source rupture and fracture orientation information to determine the main stress distribution characteristics acting on the coal pillars and assess their stability. By constructing a refined model for triggering Coulomb stress via seismic source rupture, we revealed the role of Coulomb stress accumulation in promoting coal pillar rupture. Based on the Mohr-Coulomb failure criterion, we identified the rupture tendency of the coal pillars and characterized the fracture distribution using a power-law scaling function, thus analyzing the mechanical mechanism of large deformation and instability of non-equidistant coal pillars under the influence of Coulomb stress accumulation. The results show that as the width of the coal pillar decreases, the effect of the maximum principal stress on the uniaxial compression of the coal pillar becomes more significant, shear fracture to the coal pillar occurs at a rupture face with a yield of 87°∠79°, the overall instability coefficient increases linearly, and the proportion of coefficients exceeding 0.9 increases, and the coal pillar gangs in the adjacent empty lane are deformed greatly, and the stability of the coal pillar decreases. The increase in Coulomb stress promotes the triggering of microseismicity, but there is no lower limit to the touchdown threshold, the bearing capacity of the coal pillar decreases after the width of the pillar decreases, and the cumulative effect of Coulomb stress leads to the coal pillar being more susceptible to instability. The angle between the maximum principal stress and the outer normal of the shear rupture surface of the coal pillar is 50.4°, and the coal pillar will continue to expand along the existing rupture surface, prompting a rapid increase in the Coulomb stress, which will cause the cracks in the coal pillar to penetrate, reduce the bearing strength, and the coal pillar will undergo a larger scale of damage and ultimately instability.