Abstract: During the ovet haul of Shenyang to Fushun Pipeline, it's found that pipe Corrosion was caused by the water getting into the coating. The reason why there was water in the coating is as follows: uneven foam formation of the insulating layer, vacuole formation, jacket damage, field joint of shrinkable sleeves were not sticked to the jacket to cause poot sealing; fault field joint of jacket, foaming pressure at the field joint was not sufficient and thus causing loose and low rate of hole cloosing occured.etc. And other main factors that made corrosion become worse are the existance of the reducing mycetome of sulfate and the hydrolization of flame ratardent agent in the foam as well as rather high transportation temperature. The basic reasons why the uncontrollable corrosion took place in Shen-Fu Pipeline is described, which is the vaporizing of water in foam where jacket layer is damaged. At the very position. the crystal salt on the pipewall and the soil close to the pipewall is of high resistivity to the outside due to affect by pipeline temperature. so that it prevented cathodic protection curtent from enteting, thus forming a cotrosion environment between water medium and foam and protection can never be carried out in the very atea. Through open-cut inspection, most of the corrosion was found in the middle and bottom patt of the pipe forming a belt, which suggesting that the corrosion is a typical oxygen concentration cell corrosion.