A sub-8 nm networked cage nanofilm with tunable nanofluidic channels was developed for adaptive sieving. The nanofilm, fabricated via a free-interface-confined self-assembly and cross-linking strategy, exhibits exceptional water permeability (10⁻⁵ cm² s⁻¹), surpassing traditional polymeric membranes by 1–2 orders of magnitude. The nanofilm's microenvironment can be manipulated through anion exchange, enabling control over hydrophilicity and steric hindrance. By incorporating light-responsive anions, the membrane demonstrates graded molecular sieving, effectively separating organic dyes in a single-step process. The nanofilm's unique structure allows for tunable pore sizes and adaptive permeability, making it suitable for applications in separation and filtration. The membrane's performance is stable under various conditions, including high pressure and exposure to solvents. The study highlights the potential of porous organic cages in creating bio-inspired, ultrathin membranes with advanced separation capabilities.A sub-8 nm networked cage nanofilm with tunable nanofluidic channels was developed for adaptive sieving. The nanofilm, fabricated via a free-interface-confined self-assembly and cross-linking strategy, exhibits exceptional water permeability (10⁻⁵ cm² s⁻¹), surpassing traditional polymeric membranes by 1–2 orders of magnitude. The nanofilm's microenvironment can be manipulated through anion exchange, enabling control over hydrophilicity and steric hindrance. By incorporating light-responsive anions, the membrane demonstrates graded molecular sieving, effectively separating organic dyes in a single-step process. The nanofilm's unique structure allows for tunable pore sizes and adaptive permeability, making it suitable for applications in separation and filtration. The membrane's performance is stable under various conditions, including high pressure and exposure to solvents. The study highlights the potential of porous organic cages in creating bio-inspired, ultrathin membranes with advanced separation capabilities.