Abstract: This study explores how prefabricated crack affect the mechanical properties and failure characteristics of hard rock.We conducted both laboratory experiments and numerical simulations to perform uniaxial compression tests on hard sandstone samples with varying fracture lengths and angles.By examining stress-strain relationships, fracture propagation, and acoustic emission characteristics, we found that prefabricated crack weaken hard rock in specific ways.The results show that different fracture morphologies lead to varying degrees of strength weakening in hard sandstone.Specifically, mechanical strength initially decreases and then increases as the fracture angle increases, while it consistently decreases with longer fracture lengths.Longer fractures or those angled closer to 45.0° are more likely to result in shear failure.Stress tends to concentrate at the fracture tips, causing the formation of wing and secondary fractures, which changes the splitting failure mode of intact rock and lessens the severity of sudden rock failure.The numerical simulations align with the laboratory findings on the weakening patterns caused by fractures.Using a damage variable calculation formula, we performed a quantitative analysis of the weakening and modification effects of prefabricated crack on hard rock strength.