Membranes with unparalleled solvent permeance and excellent selectivity are needed to reduce the energy input by molecular separations in organic liquids. A novel organic solvent nanofiltration (OSN) membrane with high flux was prepared from aramid nanofibers (ANFs) with in situ incorporation of microspheres. The ANF was dissociated from Kevlar aramid fibers in dimethyl sulfoxide (DMSO)/potassium hydroxide (KOH) solution and interacted with polyethyleneimine (PEI) through non-covalent interactions, forming ANF/PEI microparticles. The ANF/PEI OSN membrane was successfully fabricated by non-solvent induced phase separation and post-treatment. Dead-end filtration experiments showed that the permeance of the ANF/PEI membrane increased by a factor of over four (6.0 ± 0.5 L m⁻² h⁻¹ bar⁻¹vs. 1.4 ± 0.1 m⁻² h⁻¹ bar⁻¹). Meanwhile, the unique polymeric layered nano/micro structure of the ANF/PEI thin film membrane contributed to the ultrafast organic solvent permeance for tetrahydrofuran (THF) (20.5 m⁻² h⁻¹ bar⁻¹) and acetone (11.2 m⁻² h⁻¹ bar⁻¹). Finally, the ANF/PEI membrane significantly promotes ethanol permeance, without compromising its high selectivity when it is treated with methanol. The highest permeability was 9.1 L m⁻² h⁻¹ bar⁻¹ with an RB rejection of 98.4% after 6 d of methanol treatment. The ANF/PEI OSN membrane showed a good stability in a long-term experiment. This facile construction of nanofiber membranes with in situ modification provides a new direction for high performance OSN membranes.