The clearwater obtained from stabilized oily wastewater has become a worldwide challenge. Nowdays, the area of oil/water emulsion separation materials have accomplished great progress, but still faces the enormous problems of low flux, poor stability, and pollution resistance. Nanocelluloses (cellulose nanocrystals (CNC)) with the advantages of hydrophilicity, eco-friendliness, and regeneration are ideal materials for the construction of separation membranes. In this paper, a flexible, anti-fouling, and durable nanocellulose-based membrane functionalized by block copolymer (poly(N-isopropylacrylamide)-b-poly(N,N-dimethylaminoethyl methacrylate)) is prepared via chemical modification and self-assembly, showing high separation efficiency (above 99.6%) for stabilized oil-in-water emulsions, excellent anti-fouling and cycling stability, high-temperature resistance, and acid and alkali resistance. More importantly, the composite membrane has ultra-high flux in separating oil-in-water emulsions (29,003 L·m⁻²·h⁻¹·bar⁻¹) and oil/water mixture (51,444 L·m⁻²·h⁻¹·bar⁻¹), which ensures high separation efficiency. With its durability, easy scale-up, and green regeneration, we envision this biomass-derived membrane will be an alternative to the existing commercial filter membrane in environmental remediation.