Abstract: The intelligent pigging exposed that the asphalt coating deteriorated to a great extent in Rongshan-Foyin gas transmission pipeline. Two problems existed, one was that the unprotected area on pipe surface (mostly at 3-9 o'clock position of pipe) continuously grew up due to the asphalt coating ageing and water aggregation, especially around the circumferential welded seam of the pipe, where the protection current of CP could not be reachable; the other one was that the polarization potential of CP was insufficient. Therefore, an analysis on the external corrosion of pipe wall has been conducted, the corrosion control measures against the external corrosion of the pipeline is put forward.