The sand-filling reinforcement method is widely used in the foundation treatment of immersed tunnels. However, noise during construction on complex sites will interfere with monitoring. The application and research of real-time sand-filling monitoring and detection methods has been an important issue in civil engineering. In this paper, based on the propagation characteristics of elastic waves in a multi-layered media, a non-destructive monitoring method based on the frequency spectrum characteristics of elastic waves was presented. With the full-scale model test of immersed tunnel sand filling, the response waveform of elastic waves before and after the change of lower media was obtained, and the frequency spectrum was analyzed. The horizontal layer dielectric model of the immersed tunnel was established to get the elastic wave response waveform data of the lower medium and waveform spectrum. The change range of dominant frequency of the structure before and after sand filling was set as the filter band to reduce the interference of noise on the construction site. The monitoring results obtained by the visualization window, and those obtained by the frequency spectra, were compared, and a visualization nephogram was adopted to real-time detect the sand-filling schedule of the building foundation. This research can provide the scientific basis for sand-filling construction of immersed tunnel foundations.