An object of the present invention is to provide a method for preparing a plating solution for aluminum electroplating useful for the production of a high-ductility, high-purity aluminum foil at a high film formation rate, etc., which is an easy-to-handle plating solution that does not solidify and allows for an electroplating treatment even at 25° C. The present invention as a means for achieving the object is characterized in that in a preparation of a plating solution containing at least (1) a dialkyl sulfone, (2) an aluminum halide, and (3) a nitrogen-containing compound, the blending proportions of the dialkyl sulfone, the aluminum halide, and the nitrogen-containing compound are such that per 10 mol of the dialkyl sulfone, the aluminum halide is 3.5+n to 4.2+n mol, and the nitrogen-containing compound is n mol (wherein n is 0.001 to 2.0 mol). In addition, a plating solution for aluminum electroplating prepared by the method of the present invention allows for an electroplating treatment with high aluminum deposition efficiency relative to the current flow, and is thus advantageous in that electricity usage can be reduced, resulting in excellent economic efficiency.