Abstract: A dent leakage accident occurred in an X70 steel natural gas pipeline. By means of macro analysis, mechanical properties test, metallographic examination, micro analysis, residual stress testing and finite element simulation, the causes of pipeline dent leakage were analyzed. The results show that the pipeline was compacted by rock during laying pipelines, which resulted in pipeline dent. During the service of the pipeline, micro cracks formed in the dent due to the wall thickness of the depression was seriously thinned and the stress level rised sharply. The crack propagated gradually and penetrated the pipeline wall, eventually led to pipeline leakage.