Abstract: Five cracks and one buried defect on the girth weld of a hydrogenation reactor with service life over 50 years were found. The mechanical properties and fracture toughness of the material were obtained by continuous cyclic loading ball indentation method, and the samples were excavated from the inner wall for physical and chemical tests. It was found that the microhardness of the material was not abnormal, the grain size of the material was coarse, the microstructure was unchanged, and the toughness was good. According to GB/T 19624-2019 standard, the conventional evaluation method of planar defects was adopted, and the two mechanisms of plastic failure and fracture failure were considered. The evaluation points of five cracks and one buried defect were all located in the safety area of the double-criterion failure evaluation diagram. The results showed that the reactor could operate safely for 3 years.