Abstract: In Daya Bay nuclear plant, the type of the last stage blade of turbine rotor is MARKⅡ vertical tree root, which is designed by GEC-A. In some plants, cracks were found on the rotor blades of old type, which were not improved before by GEC-A. For that reason, Daya Bay nuclear plant began feedback preventive inspection in a very early time. But there is a problem that it has to tear down all the rotor blades before inspection each time, so the inspection will cost about 8~9 days and become the critical path of the refueling outage, whats more, it may cause the risk of mechanical damage. Therefore, to extend the cycle time of the low-pressure cylinder blades disassembly can further improve the economy of the steam turbine and keep the nuclear power plant steam turbine maintaining a reliable. Through years of researches and experiments, in-situ phased array ultrasonic inspection techniques can meet the requirement, and it has been successfully used practically.