Abstract: The ultrasonic speckle phase-stepping technique can be used to measure the displacement of some key point on inner surface of an underwater object. In performing this technique, a digital reference signal is introduced to intervene with the speckle signal picked up on a scattering surface. Then, the intensity of the added signal is obtained. Before and after object displacement, the intensity of the added signal is changed as the phase of the reference signal is shifted several times by the software. According to the intensities of these added signals, as a result, the displacement of the testing point can be measured. The out-of-plane and in-plane displacement of the inner surface of an underwater specimen were measured. The measuring data was in good agreement with the real ones. Besides, the experiment showed that a rough outer surface after being ground had no influence to the measurement. In performing this technique, complicated conversion between ultrasound and image is not needed, a whole field recording of ultrasonic speckles is not necessary, and the accuracy is no longer limited by speckle size.