Frequency Modulated Continuous Wave (FMCW) millimeter wave radar directly detects underwater sound field information from the air, providing an innovative solution for cross medium wireless communication from underwater to air. At present, there is no systematic theoretical model for the physical process of acoustic water surface micromotion, and it is difficult for radar detection to fully align with the center of water surface waves, which greatly affects communication quality. In this article, an acoustic water surface micromotion model and a diffusion model are established to conduct theoretical model simulation and experimental verification of the process of detecting the water surface by millimeter-wave radar. The results show that as the sound source level increases, the amplitude of the water surface wave increases. At the same sound source level, the amplitude and wave diffusion range of the water surface wave decrease with the increase of the sound source frequency. The signal characteristics of the experimental results of radar for water surface detection match the simulation results of the theoretical model, and an amplitude of 2.13 μm is detected, and the signal frequency is consistent with the frequency of the acoustic source, which enables the detection of acoustic water surface micromotion signals of −70 dB/Hz. The experiment has verified that the theoretical model can accurately simulate the shape of acoustic water surface micromotion, proved the feasibility of radar to detect underwater sound field information from the air, and provided a theoretical and experimental basis for building cross-medium communication links.