The invention belongs to the technical field of electrolysis, and particularly relates to a method for electro-converting magnesium chloride into high-purity magnesium oxide. The method aims to obtain a high-purity magnesium oxide product by using an extensive magnesium chloride resource with short process and low energy consumption. The method comprises the steps that a magnesium chloride solution is electrolyzed through a cation membrane electrolytic cell, CO2 gas is introduced into a cathode region and reacts with an electrolytic solution of the cathode region to directly generate magnesium carbonate, and the magnesium chloride solution is stirred while the magnesium chloride solution is electrolyzed; the electrolytic solution and the magnesium carbonate in the cathode region directionally flow through a filtering device for solid-liquid separation, and filter liquor circularly returns to the cathode region; a filtered product is dried to obtain the magnesium carbonate or basic magnesium carbonate, the magnesium carbonate or the basic magnesium carbonate are calcined to obtain the high-purity magnesium oxide product, and meanwhile, the CO2 gas is recycled; and anode gas and cathode gas are collected to obtain byproducts of hydrogen and chlorine. The method has the advantages that the magnesium chloride is directly converted into the magnesium carbonate or the basic magnesium carbonate by adopting an electrolytic method, the automation degree of the electrolysis technology is high, the purity of the obtained product is high, the large-scale production is facilitated, and the production efficiency is improved.