During the first stage excavation of the giant underground powerhouse, a collapse occurred between 0 +125-0 +147 m with the total collapse volume of 5 000 m3 and the cavity appeared like an irregular gourd shape. In accordance with monitoring result of the construction period, the formation mechanism of collapse is expounded from the five aspects of engineering geology, blasting vibration, excavation unloading, support measures as well as stress concentration. It is concluded that the primary cause of collapse formation in the underground powerhouse is the existence of vein fault β 80, while the direct cause is that deformation of vein fault β 80 caused by excavation and blasting vibration cannot be effectively controlled by support measures. So appropriate supports and reinforcement measures should be taken in the exposed surface of excavation in the zone of fracture and surrounding rock deformation monitoring should be strengthened, which is the effective means to prevent collapse occurrence.