Abstract: High pressure heater tube plate and cylinder welding joint is the final group of welding joint. The joint structure is complex and is easily susceptible to the lack of penetration. It is therefore likely to produce non-welded joint and cracks. At the same time the effectiveness of using reflection ultrasonic detection is limited by the structure influence and it can only carry on the bottom direct detection due to that the bottom condition interferers with the identification of defects. Through experimental research on the effect of workpiece size and shape on the detection ability of phased array ultrasonic, the combination of time difference ultrasonic diffraction method and phased array ultrasonic test was put forward to ensure the joint coverage. The structure identification by phased array ultrasonic inspection can also be combined to eliminate interference, which has guiding significance for similar welding joint detection in the future.