This paper introduces a method for detecting the amplitude frequency characteristics of the diaphragm structure in piezoelectric MEMS hydrophones using a reflective digital holographic microscope. The static morphology of the diaphragm structure on the hydrophone was measured using optical methods. Then, the inverse piezoelectric effect of piezoelectric materials was used to apply electrical excitation to the piezoelectric film at a certain frequency range. The dynamic three-dimensional morphology was characterized via a digital holographic microscope, and the dynamic response of the diaphragm at different frequencies was recorded to obtain key information, such as its characteristic frequency and mode shape, at the characteristic frequency. The experimental results show that this method can effectively characterize the static morphology, characteristic frequency, mode shape, and other characteristics of the diaphragm structure, which aids in accelerating the iterative development of piezoelectric MEMS hydrophones.