Abstract: This paper presents a combined method for modal identification of hydropower house structures with closely spaced modes, which can reduce mode intensity and improve the precision of modal separation by combining a reverse attenuation index window with wavelet packet frequency division. After modal separation of a signal, a random decrement technique and spare time domain method are used to identify its modal parameters. We have applied this combined method in a case study of the signals of in-situ vibrations in a large-size hydropower house structure with closely spaced modes, and compared with finite element analysis results. The calculations showed that the identification results were in good agreement with the finite element calculations and the damping ratios identified were in the normal range of reinforced concrete structures.