Abstract: As a kind of large tubular casting and forging, the stern tube shaft was usually detected by conventional ultrasonic testing method, resulting to low manual inspection efficiency and poor engineering implementation due to its large inspection area of the inner wall. Considering the C scan imaging function, the characteristics of dynamic depth focusing function and scanning speed of ultrasonic phased array testing technology, the ultrasonic phased array technique is applied to detect such stern tube shafts. Comparison with the results of conventional ultrasonic testing has proven that the designed scheme of ultrasonic phased array for stern tube shaft is feasible, reliable and has incomparable advantages.