Abstract: The cause of the leak of oil pipe line was analyzed by means of the chemical composition of base material analysis,the microstructure of welding joint and the leak source defect for the oil pipe line through physical and chemical inspection,and it was confirmed that the leak of the oil pipe line was caused by the perforation of pitting corrosion of the stainless steel. The pitting corrosion was the unique kind of rusty for the stainless steel which the individual pits are often seen on the metal surface. It was very rare that the proportion of the caliber and aperture of the corrosion pit in the essay was 1∶10 or so. The formation reason of the pitting corrosion was discussed and the environment aberration of form of the pitting corrosion was synthetically analyzed so as to arouse people′s extensive understanding on the pitting corrosion defect and to take protective measures to eliminate the hidden trouble that the pitting corrosion may bring to the engineering project.