Abstract: Pipeline safety is very important. A small defect in pipeline girth weld may lead to an accident. Automatic and accurate nondestructive testing method with fast detection speed is needed. Therefore, an ultrasonic phased array defect detection system has been developed. The deflection and focusing characteristics are analyzed at first. Then, detection principles based on geometrical acoustics approximation are described. And then, keys of system hardware construction and the software design are introduced. Application of this system is carried out on a weld test block of a big inch pipeline girth weld. The experiment results demonstrate the usefulness of such testing configuration and illustrate that the system can increase the testing efficiency.