The invention provides an automatic ultrasonic flaw detection device for pipeline welding joints, which consists of an elastic rail assembly, a carrier trolley, a probe scanning assembly and a labeling mechanism, wherein, the carrier trolley is arranged on the elastic rail assembly, the probe scanning assembly is fixed on the carrier trolley through an elastic plate, and the labeling mechanism is fixed on the elastic plate. The device has the detection technology efficiency greatly higher than that of manual ultrasonic detection in the nondestructive detection of circumferential welding joints of pipelines, has apparent advantages of quick detection speed, environmental radiation pollution reduction, labor intensity reduction and so on, effectively reduces the misjudgment of defects, can ensure that a flaw detection image is visualized, can make real-time located, quantificational and qualitative display of the defects, inevitably hold the main advantage in the nondestructive detection of marine petroleum engineering, and have good development prospect.