Acoustic and thermal comfort play an important role in the building environment. This study investigates ultra-lightweight concrete incorporating Miscanthus fibers as lightweight aggregates to improve sound absorption and thermal insulation properties. Miscanthus fibers (MF) is a kind of sound absorption biomass that can dissipate sound noise thanks to its porous and flexible inner structures and fibrous shape. However, its acoustic absorption performance in cement-based materials is rarely investigated. Therefore, the acoustic absorption and thermal insulation of ultra-lightweight Miscanthus concrete (ULMC) is investigated using two different kinds of Miscanthus fibers. Meanwhile other mechanical properties were characterized, including bulk density and flexural strength. Results showed ULMC with 30% 2–4 mm MF obtained an ultra-low density (554 kg/m 3), thermal conductivity (0.09 W/(m·K)) and high acoustic absorption coefficient (0.9) at low frequencies. It is found that the acoustic performance of ULMC can be improved by optimizing the dosage and shape of Miscanthus fibers. The developed green and sustainable bio-based ULMC possesses an excellent acoustic absorption and thermal insulation and is very suitable to use as indoor ceiling boards and in non-structural walls to make indoor living environment comfortable and energy-saving.