<p>Fissures with various sizes are present in engineering rock mass, profoundly affect the long-term stability of rock engineering; whereas, exploration on its microscopic fracture mechanisms and acoustic emission (AE) characteristics of real rock are imsufficient. Therefore, based on the triaxial test of rectangular sandstone with non-coplanar fissures, employing three-dimensional discrete element numerical model (PFC^3D), the failure mechanism and AE characteristics of rock specimens under different confining pressures and rock bridge inclinations were investigated. The confining pressure and prefabricated fissures obviously affected the crack failure behavior of the specimens. The inner cracks transitioned from tensile to shear fractures with the increase of confining pressure. Specimens with pre-existing fissures cracked at lower stresses, where macroscopic fracture surfaces extended perpendicular to the width direction, with cracks coalescing most rapidly when the rock bridge inclination was 90°. In the ligament region, the crack coalescence were divided into four patterns. As evidenced the CT results, the crack profile changes with different depths. AE simulations of fractures in specimens were performed using moment tensor inversion, which idenitifed that determining the type of fracture failure based on the R-value distribution of AE events is an effective method.</p>