Abstract: By means of macroscopic analysis, metallographic examination and energy spectrum analysis, properties and formation reasons of the overweight defects which were found by ultrasonic detection in the joints of X70 longitudinal double submerged arc welded steel pipes were analyzed. The results indicate that the defects located at the fusion line and their color and formation had a marked change compared polishing state with erosion state. There was no fish tail at the end of the defects and no inter-granular and trans-crystalline cracks at both sides of the interfacial defects as well. Carbon and oxygen content of the defect region was much higher than that of the normal area. So it can be confirmed that the defects were incomplete fusion which was caused by dirt on groove, not inclusions in the steel plate, and also not solid inclusions or cracks in the welding.