Abstract: A special fixed echo after the fist back reflection could be frequently received during inspecting quadrate aluminum forging. After carefully analyzing the refraction and reflection of ultrasonic beam of normal probe at test surface and back surface, we found that small angle incident led to wave form conversion, and wave form conversion led to a special kind of fixed delayed echo between first and second back reflection. When inspecting part with step or using double traverse technique, full attention should be paid to avoid mistreating this kind of delayed echo as flaw echo.