Abstract: In the ultrasonic transverse wave defect inspection on the weld of middle or thick plate, when the weld reinforcement is excessive, a metamophic longitudinal wave will occur, and in the certain condition, it will return to the sensor and form the false defect echo wave. Aiming at this problem, the reason of occurrence of metamophic longitudinal wave was analysed in theory, then, the condition of occurrence and the location were derived out. Associated with related example, the foregoing inference was proved. Finally, the minimal reinforcement thickness of occurring the false defect echo wave by using several sensors of common K values were concluded, it will be some guiding significance to the ultrosonic wave inspection on middle or thick plate.