Abstract: The Karst areas are widely distributed in Guizhou and rich mineral resources are distributed in these areas. Because of the high water abundance in the mineral layers and its top and bottom strata, the water hazard issues are quite critical in mine exploration. Large underground damming water body, being difficult to prevent for the high invisibility, is one of the major water bodies which cause the pit water inrush. It has important signficance for preventing the water hazard in the mining to clarify the causes and pit water inrush mechanism of large underground damming water body. Taking the 11·22 pit water inrush accident in Zhushi iron mine of Hezhang as an example, the authors made technology survey in pit water inrush. On the basis of analyzing Karst features on fault, underground run-off, presence status, water source types and various water body characteristics of pit water inrush, combining with the mine hydrogeological conditions, the authors revealed the formation mechanism of large underground damming water body in the Karst area, and clarified the characteristics and symptoms of pit water inrush. And it is concluded that water source of pit water inrush in Zhushi iron mine is large underground damming water body. The accident is caused by the Karst conduit break of large underground damming water body in excavation. The pit water inrush characteristics of large underground damming water body are unique. It is significant to carry out the work for preventing the pit water inrush, and take reasonable and feasible means in time.