AbstractHigh‐performance fiber‐reinforced composite materials demonstrate great potential for manufacturing diaphragms in human‐engineered acoustic loudspeakers. However, the notable scarcity of high‐quality fibers and the uncontrollable nature of the diaphragm structure limit the production of high‐quality sound that conforms to human hearing. In this study, a novel composite diaphragm material is devloped by integrating the swelling carboxymethyl cellulose microfiber (CMF) with the hot‐melted sheath‐core fiber (SCF) based on the “interpenetrating polymeric network” (“IPN”) strategy. Simulation methods and Flory‐Huggins theory are applied to explain the mechanism of fiber‐structure‐property interaction in composite diaphragm materials. Owing to the distinct microstructure, this bio‐based diaphragm material shows superior mechanical characteristics, including low density (≈0.92 g cm−3), high tensile strength (≈235 MPa), and high modulus (≈9.73 GPa). Moreover, the loudspeaker mounted with bio‐based diaphragm material exhibits enhanced sensitivity (≈82.6 dB) and stable performance across a broad frequency spectrum. This study not only elucidates the multiphysics working principles of loudspeakers but also establishes a crucial connection between the physical properties of diaphragms and loudspeaker performance. It opens up new avenues for the design of high‐performance bio‐based loudspeaker diaphragms in high‐fidelity (Hi‐Fi) acoustic devices.