Abstract: The tie piece boss of the last stage blade of the low-pressure cylinder of a nuclear power plant steam turbine generator unit was fractured after cumulative operation for about 72 months. The reason for the fracture of the tie piece boss of the last stage blade was analyzed by macroscopic morphology observation, chemical composition analysis, microstructure observation and hardness test. The results show that the fracture mechanism of the tie piece boss of the last stage blade was fatigue fracture, and the reason for the fracture was that the poor actual matching state of the tie piece bosses of the adjacent blades, resulting in uneven stress on the surface of the tie piece bosses and excessive local extrusion pressure. This caused the tie piece boss to bear uneven contact friction during the rotation of the steam turbine generator shaft, and under alternating loads, the fatigue cracks were initiated on the rough surface of the contact surface with the maximum stress and propagated gradually, ultimately leading to local fracture of the tie piece boss.