Hazardous dye substances discharged from the textile and dyestuff industries not only threaten local the surrounding ecosystems but are also hard to degraded. We report the preparation of process for a photocatalytic membrane device that can degrade dye pollution under visible light. This filtration membrane, with a well-organized multilayer structure, simultaneously achieves continuous and flow-through separation of degradation products. Cellulose nanofibers (CNFs) were used as a template for nanosheet C₃N₄ (NS C₃N₄) preparation; the performance for the photocatalytic degradation of dyes improved as the morphology changed from bulking to nanosheet. NS C₃N₄ was then attached to the surface of a prepared CNF membrane via vacuum filtration. This device exhibited high efficiency (the degradation rates of both Rhodamine B and Methylene blue both reached 96%), high flux (above 160 L·h^-1·m^-2·bar^-1) and excellent stability (maintaining steady flux and high separation were maintained after 4 h). This easy-preparation, easy-scale-up, and low-cost process provides a new method of fabricating photocatalytic membrane devices for dye wastewater treatment.