Abstract: The Stream turbine is the essential equipment under metal supervision and test in power plant. The fork type blade root on service is complex and its surface for defection is narrow, so it is difficult without dismantling in routine test. By adopting the ultrasonic phased array testing technology and preparing the special ultrasonic phased array transducer, a detection scheme was developed through software analysis of the data received from transducer array elements for focusing and scanning, two-dimensional imaging detection was realized for the fork type blade root. By this scheme, the crack defect could be detected quickly, directly and effectively, ensuring the safe operation of steam turbine through preventing blade fracture.