Abstract: In the steel structure of deepwater offshore oil platform, the harmful facial defects, such as the lack of penetration, the non fusion and the crack in the root, are likely to appear at the double welded T-type welded joint of medium thick plate. These defects are perpendicular to the testing surface. It is not easy to detect such defects with a single angle probe, especially for a smaller angle probe. Tandem ultrasonic testing has a good effect on vertical surface defects. The double welded T-type welded joint of deepwater offshore oil phatform was tested by tandem ultrasonic testing, the results show that this method can be used as an auxiliary detection method for conventional ultrasonic testing.