Abstract: Due to the discontinuity of water-air medium, neither the aerial electromagnetic wave nor the underwater acoustic wave can cross the water-air interface. Therefore, the communication across the water-air interface without a relay transceiver is always a research difficulty. However, the underwater acoustic wave will induce a small vibration at the water-air interface, namely water surface acoustic wave (WSAW), and the millimeter-wave radar can realize highprecision detection of micro-motion signals. Therefore, a new method for extracting WSAW signals based on cascaded imaging Frequency Modulated Continuous Wave (FMCW) radar is proposed in this paper. Firstly, the basic characteristics of WSAW and the theory of extracting the micro-motion signals with FMCW radar are studied, and secondly, the technique of underwater chirp acoustic signal combined with matched filtering technology is used to improve signal-to-noise ratio of the output signal. In response to large natural disturbances, a signal enhancement algorithm which combines the multi-angle accumulation with threshold phase compensation is put forward. Finally, the experiment is carried out under the disturbance of different wave heights, which verifies the effectiveness of the method proposed in this paper. It also indicates that the WSAW signal can still be extracted successfully under the natural wave with 1.5 cm wave height, and verifies that this system can also communicate effectively when the imaging FMCW radar deviates from the horizontal position, which validates the effectiveness of the proposed method.