Abstract: To investigate the fracture cause of GH6783 medium-pressure main steam valve bolts, this study employed methods including fracture analysis, metallographic testing, room-temperature mechanical property testing, and chemical composition analysis for examination. The results indicate that the direct cause of the bolt fracture lies in two aspects: excessive preload on the bolts, and continuous exposure to vibration and alternating thermal stress during installation, commissioning, unit start-stop, and operation processes. Meanwhile, the fundamental cause of the bolts' intergranular brittle fracture is the segregation in their metallographic structure—there is a large amount of banded β-phase distributed in an intergranular or transgranular manner within the structure, which ultimately leads to a significant reduction in the solid solution strengthening effect of the β-phase.