Abstract: Macroscopic and microscopic morphology observation, physical and chemical properties of pipe material test, hydraulic blasting test, casing type analysis and strength rechecking of casing string were performed for a piercing casing pipe. The failure reasons were analyzed by combining relative theory with tests. The results show that the dint on the outer surface of piercing casing was a fold defect which was the primary reason for the piercing failure of casing pipe. And the pipe body cracking was local overload cracking.