Abstract: Tube to Safe-End joint in Reactor Pressure Vessel(RPV) belongs to Austenite-Ferrite type Dissimilar Weld. It is difficult to perform UT because of its coarse grain structure. Considering deficient detectivity of planar defect by angle longitudinal wave technical, a straight longitudinal wave technical which takes Safe-end as scanning surface is put forward based on Joint structure. Experiment and research indicate it has good detectivity of planar defect which is vertical to the weld surface. This technical may be applied in manufacture stage of RPV Combining with angle longitudinal wave technical.