The present invention is to provide an in-mold foaming method for making sole, with which the sole can be produced quickly and easily, which includes the procedures as 1) inputting foaming material in mold, wherein a volume percentage of foaming material in cavity of the mold is pre-calculated based on pre-designed specific gravity; 2) in-mold foaming; 3) cooling the foamed sole; 4) semi-finished product of sole is completed. With the procedures 1) inputting foaming material in mold and 2) in-mold foaming, the procedures of air-circulating oven post-cure, cooling and cutting procedures can be omitted, so as to simplify the technical process and improve the working efficiency.