The invention discloses an aluminum-gallium separation method for a process of extracting an aluminum oxide from coal ashes employing an acid method. The method comprises the steps of adding N,N-dimethyl dithiocarbamate [(CH3)2NCSSM.2H2O] to an acid water solution of an aluminum salt at the molar ratio of Fe<3+> to M being 1 to (0.5-6) under agitation step by step, controlling the reaction temperature to be 5-40 DEG C and generating black sediments in reaction; and carrying out solid-liquid separation to obtain an aluminum-containing solution and gallium-containing precipitated slag charge, wherein the acid water solution of the aluminum salt is an aluminum hydrochloride solution which is obtained through extracting the aluminum oxide from the coal ashes through acid treatment and carrying out solid-liquid separation to remove solid residues. According to the separation method, the highest extraction rate of gallium can reach about 93.24% and the loss rate of aluminum is very low and smaller than 1%.