Abstract: 15-5PH stainless steel forging cracked after only 30 h of service, and the failure causes were analyzed by macroscopic fracture analysis, metallographic observation, scanning electron microscope (SEM) fracture analysis, chemical composition testing, and mechanical property testing, in order to ensure the safe and reliable operation of similar critical components. The results showed that no excessive inclusions that might cause cracks was detected by metallographic examination. The measured value of each chemical component in the sample met the reauirements in national standard GB/T 1220-2007 for 15-5PH steel. The cracking originated from the inner surface of ϕ1.5 mm through-hole, mainly due to the combined effect of the excessive surface roughness (R_a≥10.0 μm) of the inner surface of the hole and assembly residual stress, which resulted in significant stress concentration, inducing multiple fracture sources and continuous propagation. The measured lateral impact energy of the material ranged from 16.0 to 22.6 J, with some values below the standard requirement of 20.3 J specified in GB/T 1220-2007. The insufficient impact toughness further accelerated crack propagation, ultimately leading to failure of the forging.