Abstract: At present,offshore platforms are welded with steel material.The main weld seams are tubular joint welds and have very complicated shape,which are ditticult to realize automatic testing for defect in contrast with the girth welds,longitudinal welds and twist weld seams.An ultrasonic phased array probe scanner with three degrees of freedom was designed and made based on the specialities of tubular joint welds on offshore platform.The pose relation between the scanner and tubular joint weld was set up,the concrete expression methods of pose matrix between phased array probe and weld seam and scanner base were given.The pose matrix method provided automatic control foundation to scanner.The automatic testing to complicated shape welds of tubular joint could be realized by the scanner.The experiments on reference block with the scanner were carried out and the results indicates that the artificial defect on the reference block can be truely inspection.It can satisfy the requirement of automatic testing to tubular joint welds.