The invention belongs to the field of nuclear fuel reprocessing, and in particular relates to a device and a method for preparing tetravalent uranium by dynamic membrane electrolysis. The device consists of a cation exchange membrane, an anode, a cathode, anolyte, catholyte, a peristaltic pump, a gasket a, an electrode frame a, an anolyte outlet, a catholyte outlet, an anolyte inlet, a catholyte inlet, an electrode frame b and a gasket b, wherein the electrode frame a, the gasket a, the cation exchange membrane, the gasket b and the electrode frame b are arranged on the device from left to right and superposed layer upon layer to form a sealed space; each of the gasket a and the gasket b has a quadrilateral structure; the anode is a titanium-coated ruthenium iridium mesh; the cathode is a titanium mesh; the distance between the cathode and the anode is 6 to 15mm; and the cation exchange membrane is a perfluorosulfonated cation exchange membrane. By the device and the method for preparing the tetravalent uranium by dynamic membrane electrolysis, the content of the tetravalent uranium in a product is ensured, and the tetravalent uranium can be stably and quickly prepared; and the device has a simple structure.