Abstract: The medium-pressure rotor moving blade of steam turbine in a power plant was cracked during use. The reasans for moving blade cracking were analyzed by means of macro morphology analysis, chemical composition analysis, mechanical property analysis, microstructure observation and fracture micro area analysis. The results showed that cracks of moving blade were distributed along the grain and there was obvious full decarburization in microstructure on both sides of cracks, indicating that the cracks formed in hot working stage, which was typical original forging cracks. The phosphorus content in the material exceeded the standard. It was easy to form low melting point brittle eutectic products, and intergranular cracks were easy to form at grain boundaries of material during forging process. Under the action of working load, the cracks gradually spread and finally caused the frature of moving blade.