This paper aims to investigate the mesoscopic fracture mechanisms of sandstone under triaxial compression. We conducted triaxial compression tests of sandstone and integrated the Grain-Based Model (GBM) and moment tensor theory to simulate the development characteristics of internal cracks, acoustic emission events, and fracture strength in sandstone. The derivation process of microcracks and the characteristics of acoustic emission response in sandstone are derived, and the spatiotemporal evolution process of cracks and acoustic emission events at the micro scale is comprehensively analyzed. The results indicate that microcracks are randomly distributed in the sandstone sample during the initiation stage of fracture under triaxial compression, and the displacement field shows a horizontally layered distribution. As the loading increases, the number of microcracks increases, penetrates gradually into two macroscopic cracks, and shifts the displacement field to a significant heterogeneity pattern. The cracks in the incubation period show slow development, and cracks grow at extremely high nonlinear rates. The order of crack development follows a sequence of intergranular tensile cracks, intergranular shear cracks, intragranular tensile cracks, and intragranular shear cracks. The simulation analysis revealed that the majority of internal cracks triaxial compression (83.2%) are tensile cracks resulting from tensile failure. The number of acoustic emission events in the samples has a negative exponential correlation with the number of cracks, and a single acoustic emission event generates 75.60% of microcracks. Finally, the analysis of acoustic emission events, the number of microcracks, and the fracture strength indicates that the three factors approximately follow a normal distribution.