Abstract: In a 660 MW ultra-supercritical machine of a coastal power plant, cracking occurred at the end of the secondary blade fork-type blade root pins of the A and B low-pressure rotors, and the cracking causes of the pins were studied by macro/micromorphology observation, chemical composition analysis, microstructure observation and mechanical property tests. The results show that stress corrosion cracking occurred in the pin. The microstructure of cracking pin steel was uneven, and macroscopic coarse grains existed. The stress corrosion sensitivity of the material was higher. Corrosive media existed in the pin hole, which caused corrosion pits forming on the surface of the pin. The corrosion products contained chlorine, sulfur and other elements. In the process of high-speed rotation of the steam turbine, the pin bore the bending stress caused by the centrifugal force of the blades, and microcracks germinated at the corrosion pit. Stress corrosion cracking occurred.