Abstract: Fissures exert a significant influence on the stability of deep rock mass engineering. To investigate the influence of fissure length on the mechanical behavior of sandstone under triaxial compression and the fracture mechanism, this study adopted a combined approach of triaxial compression tests and acoustic emission (AE) monitoring. The effects of fissure length on the mechanical behavior and crack propagation characteristics of sandstone were analyzed. The evolution of micro-cracks and contact force distribution in the specimens was investigated using PFC3D discrete element simulation, and the fracture mechanism was further explored. The results show that fissure length exerts a systematic influence on the macroscopic mechanical properties,failure mode,crack evolution,and micro-damage mechanism of sandstone. Compared with the intact specimens,the peak strength of sandstone with prefabricated fissure lengths of 10,20,and 30 mm decreased by 26. 0%,38. 4%,and 63. 8%, respectively,while the elastic modulus decreased by 1. 5%,17. 1%,and 35. 7%,respectively. As the prefabricated fissure length increases,the failure mode of sandstone changes from “typical brittle failure” to “progressive,multi-stage failure,” and the RA value can effectively characterize the degree of shear failure. The increase in fissure length causes the force chain to evolve from a uniform distribution throughout the specimen to a high concentration at both ends of the fissure. The stress concentration effect is significantly enhanced,which accelerates the instability of the force chain.