The purpose of this paper is to investigate the failure cause of a stainless steel pipe for carrying marsh gas. Experimental analysis methods included examination of appearance, chemical composition analysis by spark, metallography observation by optical microscopy and scanning electron microscopy, fractured surface analysis, corrosion product analysis, oxalic acid electrolytic test and observation of microorganism. The analysis results showed that there were different size cavities on the inside wall surface, some chlorides inside the pitting hole of the section, incomplete fusion and misalignment in the weld, normal microstructures of substrate and weld, and no sensitization of substrate. From the results, we speculated that the leakage of stainless steel pipe should be resulted from pitting by chloride which came from waste hydrochloride acid for pickling. Pitting and weld defects both accelerated the perforation of weld, and finally leaded to leakage.