The invention discloses a preparation method of molasses retarding water reducer and an application thereof, and belongs to the field of concrete admixtures. Molasses undergoes oxidation until the content of carboxyl accounts for 1.5-4.0% of the molasses solid quality. The preparation method provided by the invention is simple, easy to operate, and requires short reaction time; the product prepared by the method has good dispersion and dispersion conservation with the water reducing rate being 10-15% and with no generation of insoluble substances as well as a precipitate when compounded with naphthalene.