Abstract: The cause of a special echo generated on adusting the equipment for the test of small-diameter tubes was analysed in the paper. The results showed that, the special echo was received by transducer after the ultrasonic wave was reflected for several times, and the propagation path of echo was very comlex when carrying out ultrasonic test. Technicians should not only interpreter the echoes according to experience, but should also calculate and analyse any suspicious echoes to avoid any misdetection. Meanwhile, the technological level of technicians can be improved greatly in the process of calculating and analysing.