Biomass fibers are recognized as degradable and renewable green natural polymer materials. However, research on biomass fiber-based lithium battery separators is still relatively limited. Here, a kind of cellulose diacetate-based composite separator for lithium batteries was investigated via electrostatic spinning technology. In this study, M-CDA/PEO/BM composite separators were obtained by modifying fiber-based nonwovens through deacetylation, followed by an infiltration process and thermal cross-linking reaction to introduce boehmite (BM) and polyethylene oxide (PEO). Wherein the infiltrating solution is composed of polyethylene glycol diglycidyl ether (PEGDE), polyetheramine (DPPG), and boehmite particles. The results demonstrate that this nanofibre composite separator exhibits good heat tolerance (no shrinkage at 200 °C), excellent wettability (contact angles only 15.1°), and high ionic conductivity (2.83 mS cm−1) in electrolytes. Additionally, the cells utilizing the composite separator exhibited superior cycle performance and better multiplier performance (170 mAh·g−1 at 0.5C) compared to the polypropylene separator that is currently available in the market, under identical circumstances. This renewable composite possesses remarkable properties that position it as a highly promising option for lithium-ion battery separators with exceptional performance.