Compared with traditional vibration measurement techniques, ultrasonic speckle stroboscopic technique can perform on-line, underwater, noncontact experiments, and is insensitive to the environment and the sample surface roughness. In this manuscript this technique was applied to a vibrating cantilever underwater. The experimental results were in good agreement with other testing methods. Therefore, the noncontact testing technique for vibration coefficient, especially the vibration phase difference, provides an alternative method for the mode analysis of industrial constructions, which is a piece of very important work for industrial underwater structure design.