Abstract: During the manufacture of reactor pressure vessel (RPV), a large area of continuous weld defect was found near the fusion line of the inner surface of head flange after ultrasonic testing (UT) of the component. The defect was crossing more than 8° in the circumferential direction, resulting in a wide range of thinning of base metal for low alloy steel. The root cause of the above defect was analyzed and weld repair scheme of repaired stainless steel was given according to the present technical situation. Detailed mechanical analysis was carried out for the repaired structure. Influence of the defect on the performance of the RPV was analyzed from the view of stress, fatigue and sealing. The feasibility of the repair scheme was demonstrated. The scheme of repaired stainless steel is applied in engineering, which can provide experience for solving similar engineering problems.