Resonant sound absorption metamaterials have excellent noise control capability and great potential for application as a lightweight aerostructure, but challenges remain in low-frequency broadband noise control. In order to promote the synergistic design of low-frequency and broadband sound absorption of metamaterials, a design method for resonant sound absorption metamaterials with a triangular back cavity in a perforated plate is proposed and the acoustic properties are investigated. The metamaterial divides the honeycomb cavity into six independent triangular units, and microperforations are arranged in the center of each triangular unit to achieve broadband sound absorption by using the coupling effect between the units. The effects of hole diameter, depth and height on the absorption coefficient and peak frequency are investigated, and the multi-unit coupling mechanism of the acoustic metamaterial’s broadband sound absorption is analyzed in terms of the zero-pole distribution of the complex frequency plane and the energy dissipation of the metamaterial. The low-frequency broadband sound absorption metamaterials designed according to the units and their coupling characteristics achieve more than 80% absorption in the low-frequency range of 309–464 Hz, with a thickness of 50 mm, which is only 1/22 of the longest working wavelength, and an equivalent density of about 0.31 g/cm³, which realizes materials’ lightweighting. The structure was prepared by light-curing molding technology and its acoustic performance was experimentally verified in an impedance tube. This lightweight sound absorption metamaterial design provides support for low-frequency noise control of equipment.