The invention belongs to the technical field of ultrasonic detection of austenitic stainless steel welded seams, and specifically relates to an ultrasonic flaw detection method of heavy-gauge austenitic stainless steel welded splices. The method adopts longitudinal-wave double-crystal angle probes to conduct a method of oblique wave and direct wave from two sides on one side of welded seams of an austenitic stainless steel with the thickness less than 163mm. During the process of scanning, the welded seam is laminated in the direction of thickness and the combination in every layer uses probes in different focal lengths and angles to detect, meanwhile, an overlapped area is positioned between two adjacent layers of probes. By testing capabilities of the detection system, according to the technical scheme, the detection system fully meets the ultrasonic detection requirements of welded seams of heavy-gauge austenitic stainless steels which are the preparation materials of spent fuel transfer casks.