Abstract: Concrete-filled steel tube (CFST), a composite structure widely used in bridges and high-rise buildings, may develop void defects between the internal concrete and the steel tube due to construction quality issues or concrete shrinkage, leading to a significant reduction in structural bearing capacity. To effectively evaluate such defects, this study conducts non-destructive testing research based on the air-coupled impact-echo method. A simplified 2D finite element model of CFST was established, and experiments were performed on large-scale specimens containing polyurethane (PU) voids of varying sizes. The power spectral density (PSD) characteristics of the impact-echo signals were collected and analyzed. The results show that the integrated area under the PSD curve can effectively characterize the presence and size variation of voids, enabling reliable assessment of void conditions in CFST structures. The findings of this study provide methodological guidance and experimental evidence for the non-destructive evaluation of CFST structures.