The invention relates to a dispersing agent for a fluorescent brightener, a fluorescent brightener aqueous dispersion, a preparation method of the dispersing agent, as well as a preparation method and application of the fluorescent brightener aqueous dispersion. The preparation method of the dispersing agent comprises the following steps: firstly, separately dissolving acrylamide and an allyl polyoxyethylene ether monomer of which the polymerization degree is 23 in a proper amount of deionized water; then, controlling the temperature to be 79-81 DEG C under the stirring condition, separately adding the allyl polyoxyethylene ether monomer aqueous solution and an ammonium persulfate initiating agent into a four-mouth flask filled with a certain amount of deionized water drop by drop within 2-3 hours, and carrying out 3-4 hours' heat preservation after the drop-by-drop adding is finished; finally, carrying out cooling to prepare the dispersing agent of which the weight-average molecular weight is 10000-20000. The invention further provides the fluorescent brightener aqueous dispersion which contains the fluorescent brightener OB-1, the prepared dispersing agent and deionized water. When the fluorescent brightener aqueous dispersion is applied to whitening of a fabric, the fabric can be whitened and brightened very uniformly, and the whitening effect is obvious.