Abstract: In order to enhancethe sizing accuracy for defect inspected of the main steam pipeline weld of the HPR1000 during the in-service stage, the portable automatic ultrasonic TOFD testing system is applied to inspect the circumferential weld of the low alloy steel factory product with thickness of 52 mm. The inspection results show that the ultrasonic TOFD technique can be well adapted to the detection of thick-walled welded structures, and has a high sensitivity. By analyzing the problem of the near-surface dead zone with the ultrasonic TOFD technique, the automatic in-service inspection strategy for the welds of main steam pipelines’ welds in HPR1000 nuclear power station is proposed, which is combined with the ultrasonic TOFD technique and the conventional pulse reflection technique, so as to achieve the purpose of accurate sizing and characterization of defects, and integrity evaluation of the weld structure.