The movable guide vane of domestic large hydropower units in early stage was mostly made by sand casting, which had the characteristics of large abnormal shape parts and they were located in the water wheel room of the units with narrow space. At present, the nondestructive testing of the area where the movable guide vane is hided in the shaft sleeve of large hydropower unit in service is generally carried out after the movable guide vane is lifted out during the unit overhaul period. This paper attempts to test it though phased array testing technology, modeling simulation and simulated test block test. This method is successfully applied in actual detection. Finally, the problem that coarse crystal, long sound path and abnormal shape of casting movable guide vane have serious attenuation, loss of sound energy, and difficulty in receiving defect echo is solved. The feasibility of nondestructive detection on the hided area of casting movable guide vane in completely assembling state of large hydropower units in service is verified.