To systematically investigate the influence of axisymmetric double fractures with varying lengths on the mechanical properties and failure mechanisms of the internal bearing capacity of rock boreholes, two distinct types of sandstone specimens, characterized by differing inherent strengths, were fabricated. Specimens were prepared in two configurations: one set without fractures and another set with axisymmetric double fractures measuring 10, 20, and 30 mm in length, respectively. A specialized loading method was developed for the cracking test within the borehole, and digital image correlation (DIC) technology was employed to monitor the deformation and fracture processes. Based on the mechanical parameters and failure characteristics derived from laboratory tests, this study further examines the internal bearing mechanical behavior of specimens with different prefabricated crack lengths under true triaxial stress conditions, utilizing the PFC3D particle flow simulation program. The results indicate that prefabricated fissures significantly reduce the bearing capacity of the samples and induce directional fracturing. An optimal fissure length exists; beyond this point, further increases in fissure length result in an exponential decrease in bearing capacity. Notably, samples with higher lithological strength exhibit a more gradual attenuation trend. During the loading process, the vertical strain field shows a time-sequential characteristic, where the strain value in the fracture direction initially increases and remains dominant. Sudden changes or transitions from tension to compression in strain values can serve as precursor indicators of imminent sample failure. Under true triaxial stress conditions, a principal stress difference coefficient of 0.5 serves as the critical threshold for internal tensile bearing performance. When this coefficient is greater than or equal to 0.5, the minimum principal stress has a minimal influence on bearing performance, allowing for the use of fracture mechanics plane models to approximate the bearing behavior of boreholes with prefabricated fissures.