The sound absorption mechanisms of underwater sandwich composite cavity structures encompass flexural vibration of the cover plate, radial vibration of the cavity walls, and cavity resonance. In this paper, a series of sandwich composite cavity structure samples were fabricated, and the resonant sound absorption characteristics under air-backed and water-backed conditions were investigated using the impulse tube test method. The experimental results indicate that as the cavity depth and diameter increase, the resonant sound absorption peak frequencies shift to lower frequencies and the bandwidth decreases under both backing conditions. When the cavity spacing decreases, the first-order resonant sound absorption peak increases and the peak frequency shifts to lower frequencies under air-backed conditions, whereas the opposite occurs under water-backed conditions with an increase in bandwidth. The peak frequency and sound absorption bandwidth of cavity resonant sound absorption are proportional to the cavity volume. Under both backing conditions, the first-order resonant sound absorption peak frequency of cylindrical cavities is the lowest, with the narrowest sound absorption bandwidth, whereas conical cavities exhibit the opposite trends.