Abstract: This paper conducts a systematic investigation on the fracture incident of Inconel Alloy 783 bolts in the intermediate pressure main steam valve of a 660MW ultra-supercritical unit. During operation in June 2022, 29 IP main stop valve bolts fractured, with examinations revealing that the fractures predominantly occurred at the thread root of the studded ends.Through multi-scale analytical methods including macroscopic morphology observation, metallographic analysis, scanning electron microscopy , and mechanical property testing, the study identified distinct fracture characteristics: Bolt No. 16 exhibited intergranular fracture features with a rock-candy morphology, displaying a microhardness of 370-375 HBW (exceeding relevant standard limits) and evident oxidation in the crack propagation zone; Bolt No. 18 demonstrated a transgranular-intergranular mixed fracture pattern, initiating from the stress concentration area at the thread root and showing a fragmented grain transition zone during crack propagation.Finite element analysis revealed a stress concentration factor of 3.2 at the thread root, confirming that excessive preload was the primary trigger for crack initiation. Based on these findings, improvement measures were proposed, including optimized preload processes, enhanced thread geometry, and preferential adoption of advanced materials such as 1Cr11Co3W3NiMoVNbNB.This research provides critical experimental evidence for material selection, design optimization, and lifespan evaluation of high-temperature fasteners in ultra-supercritical units.