Osmotic energy stored between seawater and freshwater is a clean and renewable energy source. However, developing high-efficiency and durable permselective membranes for harvesting osmotic energy remains a longstanding bottleneck. Herein, we report that a nanocomposite membrane with a biological serosa-mimetic structure can achieve high-performance osmotic energy generation through the coupling of two-dimensional (2D) sulfonated covalent organic framework (COF) nanosheets and anion-grafted aramid nanofibers (ANFs). As verified by theoretical calculations and experimental investigations, the 2D COF nanosheets not only provide abundant one-dimensional (1D)/2D nanofluidic channels to synergistically benefit an ultrafast ion migration but also enable high cation permselectivity via the covalently tethered anions. The grafted ANFs increase the mechanical strength of the membrane and further improve the ion diffusion/rectification. When it was applied in an osmotic power generator, the biomimetic membrane delivered a power density of 9.6 W m^-2, far surpassing the commercial benchmark of 5.0 W m^-2. This work could boost the viability of osmotic energy conversion toward a sustainable future.