Assembling　two-dimensional　(2D)　materials　by　polyelectrolyte　often　suffers　from　inhomogeneous　microstructures　due　to　the　conventional　mixing-and-simultaneous-complexation　procedure　(＂mix-and-complex＂)　in　aqueous　solution.　Herein　a　＂mix-then-on-demand-complex＂　concept　via　on-demand　in　situ　cascade　anionization　and　ionic　complexation　of　2D　materials　is　raised　that　drastically　improves　structural　order　in　2D　assemblies,　as　exemplified　by　classical　graphene　oxide　(GO)-based　ultrathin　membranes.　Specifically,　in　dimethyl　sulfoxide,　the　carboxylic　acid-functionalized　GO　sheets　(COOH-GOs)　were　mixed　evenly　with　a　cationic　poly(ionic　liquid)　(PIL)　and　upon　filtration　formed　a　well-ordered　layered　composite　membrane　with　homogeneous　distribution　of　PIL　chains　in　it;　next,　whenever　needed,　it　was　alkali-treated　to　convert　COOH-GO　in　situ　into　its　anionized　state　COO--GO　that　immediately　complexed　ionically　with　the　surrounding　cationic　PIL　chains.　This　＂mix-then-on-demand-complex＂　concept　separates　the　ionic　complexation　of　GO　and　polyelectrolytes　from　their　mixing　step.　By　synergistically　combining　the　PIL-induced　hydrophobic　confinement　effect　and　supramolecular　interactions,　the　as-fabricated　nanofiltration　membranes　carry　interface　transport　nanochannels　between　GO　and　PIL,　reaching　a　high　water　permeability　of　96.38　L　m-2　h-1　bar-1　at　a　maintained　excellent　dye　rejection　99.79%　for　150　h,　exceeding　the　state-of-the-art　GO-based　hybrid　membranes.　The　molecular　dynamics　simulations　support　the　experimental　data,　confirming　the　interface　spacing　between　GO　and　PIL　as　the　water　transport　channels.