Covalent　organic　frameworks　(COFs)　are　typically　isolated　as　microcrystalline　powders.　It　remains　fundamentally　challenging　to　fabricate　COFs　into　high-quality　self-standing　films　to　take　full　advantage　of　their　ordered　pore　channels　for　molecular　separation.　Here,　we　report　a　new　strategy　for　fabricating　self-standing　imine-linked　COF　films　via　homogeneous　polymerization　where　films　emerge　from　clear　solutions　without　forming　amorphous　precipitates.　The　abundant　basic　nitrogen　atoms　of　the　monomers　acted　as　a　reaction　controller　to　realize　the　homogeneous　polymerization　and　also　promoted　the　tight　self-aggregation　of　COF　crystallites　to　form　compact　films　via　H-bonding.　The　chemically　supported　self-standing　COF　films　on　nylon　membranes　were　also　developed　via　an　in　situ　growth　method.　The　resulting　films　showed　an　unprecedentedly　ultrafast　permeance　of　2822　L　m-2　h-1　MPa-1　with　a　high　rejection　rate　(99.8%)　in　the　filtration　of　a　congo　red　(CR)　solution,　demonstrating　the　advantage　of　this　new　strategy　in　fabricating　high-quality　self-standing　COF　films.　Copyright　©　2020　American　Chemical　Society.