<p>Sandstone and mudstone frequently present interbedded slope structures. Within theses formations, inherent fractures and bedding planes can undergo gradual cracking and achieve connectivity. This occurrence gives rise to potential failure surfaces, thereby posing a risk of landslides. To comprehend these failures, it is essential to investigate the initiation and propagation processes of cracks within sandstone or mudstone. A series of uniaxial compression tests and numerical simulations were carried out on samples with pre-existing fissures at different angles. The Digital Image Correlation (DIC) technology was utilized to monitor the macroscopic crack propagation during the testing process. A comparative analysis was conducted on the crack modes and initiation angles observed in both rock types. The results suggest that the crack propagation behaviors of sandstone and mudstone under single and double pre-existing fissures are largely similar. For both rocks, the initiation angle of cracks decreases as the angle of the pre-existing fissures increases. In sandstone, the influence of shear forces on crack propagation becomes increasingly prominent, while in mudstone, tensile forces prevail as the cracks develop.</p>