In response to the failure of the last stage blades of the low-pressure rotor of a power plant steam turbine, macroscopic inspection, chemical composition analysis, mechanical performance testing, metallographic inspection, and microscopic analysis of the fracture surface were conducted. Its the first relevant study on the failure mechanism of the quenched layer of the last stage long blade in long-term operation. Study the failure mechanism of the last stage blades. The results indicate that the failure mechanism of the final stage blade is stress corrosion cracking. The main reason for blade failure is the presence of a small amount of chloride ions in the steam medium, and the high strength of the quenched layer is very sensitive to stress corrosion. Under centrifugal tensile stress and vibration, the quenched layer undergoes stress corrosion cracking. After the crack extends to the base material, the sensitivity of the base material to stress corrosion decreases, and the crack mainly propagates in a fatigue manner.