Abstract: Foreign body blockage already became the main reason of super-critical boiler tube explosion,and researchers pay attention to its testing.By simulating the actual working environment,various objects were placed in the water filled small diameter tube to simulate the different types of blockages and ultrasonic longitudinal wave was used to test the blockage.The results show that there is a blockage if the probe wall echo by the side wall of the tube decreases significantly or is to disappear.If there is no blockage,the side wall echo is certainly to appear in the acoustic path and its amplitude shows minor change.Therefore,the existence of the blockage in the pipe can be determined according to the tube wall echo amplitude variations.