Abstract: Ultrasonic testing is a common method for welds. There is obvious heterogeneous interface echo of the conventional ultrasonic testing for stainless steel-ordinary carbon steel flash welding, which will seriously affect the defect recognition because it is difficult to distinguish the defect signal in the heterogeneous interface ones. In this paper, sound pressure reflection coefficient of the heterogeneous interface is tested, aiming to illuminate the reason of existence of the reflection echo.