Abstract: A Model assisted probability of detection (MAPOD) method used for reliability analysis of the ultrasonic testing procedure with different angle beam probes for the weld of step-like irregular structure in the power unit is developed based on the ultrasonic numerical model and POD model. The proposed method is used to analyze the conventional inspection procedure of main steam pipe with steps and the result shows that there is an uncovered zone by the composition of K1 and K2 probes. The depth of the uncovered zone is 12 mm when the thickness of pipeline is 89 mm. The echo of defect on the top of weld is influenced by the steps of the weld of irregular structures, and the size of the lack of fusion must be greater than 5 mm to guarantee the 90% of POD.