During the excavation and unloading of underground chambers, the mechanical characteristics and crack propagation behavior of jointed rock formations are vital for project safety and stability. In this study at the Karala Hydroelectric Station in Sichuan, we conducted comprehensive tests on non-penetrating jointed sandy shale under triaxial loading and unloading conditions. We analyzed the mechanical properties and deformation characteristics under different stress paths, explored the applicability of various strength criteria, and investigated crack propagation mechanisms using fracture mechanics. The research outcomes have yielded the following insights:(1) As initial confining pressure increases, axial stress and strain at failure show an upward trend under different stress paths.(2) Compared to triaxial loading, unloading and jointed samples decrease load-bearing capacity. Unloading from axial stress to confining pressure increases cohesion (c) by 4.1% and decreases it by 30.4% during unloading from confining pressure to axial stress. Internal friction angle (φ) increases by 3.5% and 7.3% during axial compression, while jointed samples show a decrease of 32.9% and 53% in cohesion (c) and a decrease of 2.2% and 10% in internal friction angle (φ).(3) In terms of characterizing the strength properties of sandy shale throughout the loading and unloading process, the Mogi-Coulomb strength criterion outperformed both the Mohr-Coulomb and Drucker-Prager criteria. (4) The theoretical critical angle increases with confining pressure, and biconjoint samples exhibit higher values than intact ones. The theoretical critical angles range from 55° to 60° under different loading paths.