Covalent organic framework (COF) membranes with tailored functionalities hold great promise in diverse applications, but the key to realize their full advantages of highly ordered pore structures is the development of membrane fabrication approaches. In this work, we report a potential difference-modulated biphasic strategy to fabricate large-area, self-standing COF membranes under ambient conditions. The fabrication was conducted at the polarized water/1,2-dichloroethane (water/DCE) interface, where HCl was dissolved in water as a catalyst and monomers (both amine and aldehyde) were added to DCE. The external polarization of the water/DCE interface by cyclic voltammetry can continuously pump H + from water to DCE to boost the Schiff base reaction of monomers and the growth of COF membranes. Moreover, the growth process can be real-time-monitored by interfacial double-layer capacitance measurement, and the permeability of COF membranes can be in situ-examined by heterogeneous ion transfer voltammetry. Given that the potential difference across the water/DCE interface can be also facilely modulated by dissolving proper electrolyte ions in two phases, the fabrication of large-area COF membranes is made possible in beakers. Using this strategy and different monomers, three types of centimeter-scale, free-standing COF membranes with tunable pore size and surface functionality were prepared, and their defect-free structure was proved by the molecular permeance and ultrafiltration test. We believe that this biphasic strategy offers a controllable and scalable way to fabricate COF membranes and sheds light on development of novel self-supporting membranes with unique functions.