<p>Rock pillar burst during underground mineral resource digging has seriously endangered worker safety in underground engineering construction. In this study, to enhance the understanding of fragmentation characteristics, a series of four-face unloading rock pillar burst tests were designed and conducted under different confining pressures and the characteristics of rock fragments and cracks were analysed. The results indicate that confining pressure has an important effect on the morphology of rock fragments and the intensity of rock pillar burst is greater than the intensity determined using a uniaxial compression test. The rock fragments can be divided into four categories: coarse grain, medium grain, fine grain and micro grain. The rock fragments were long-strip and spicule shaped. The equivalent size of rock fragments decreased gradually and the fractal dimension of size–frequency, box fractal and density of cracks increased gradually with increasing confining pressure, indicating that when rock fragments were smaller, more energy was released and the intensity of the rock pillar burst was more violent. The mass percentage of rock fragments was fitted using the Weibull distribution, and the scale parameter <i>k</i> decreased gradually with increasing confining pressure. A general positive linear correlation exists between the fractal dimension and dissipation energy density, but a general negative linear correlation exists between the fractal dimension and the scale parameter <i>k</i>. The Mohr-Coulomb theory is introduced to explain the failure mechanism of rock pillar burst. Some engineering cases are collected and a comparison analysis is conducted between actual engineering and experimental results. The experimental results are consistent with the engineering. The research results have a certain reference value for the comprehensive understanding of the four-face unloading rock pillar burst in underground engineering.</p>