Abstract: The steam turbine is one of the most important rotation equipments in the Nuclear Power Plant, the blade of steam turbine is likely to engender crack in the root position that may eventually affect the safety work of low-pressure rotor. The article mainly introduces ultrsonic phased array technology on methods concerning the blade of steam turbine. It is helpful to master the method on how to judge the defects according to the inspection technique of Mushroom shaped leaf root. The inspection result indicated that present technique had good application effectiveness, and the development of the ultrsonic phased-array technology of blade root had great significance in the area of assuring the safety and reliability of running condition of steam turbine.