The invention discloses a method for preparing a polyhydroxy auxiliary agent for fly ash fiber papermaking, which is characterized by comprising the following steps: firstly, adding 20 to 30 percent of mixed monomer aqueous solution by total weight of the mixed monomer aqueous solution into a reaction kettle, heating the reaction kettle to 70 DEG C, adding 10 to 20 percent of initiator aqueous solution by total weight of the initiator aqueous solution into the reaction kettle, and preserving the heat for 20 to 30 minutes at 70 DEG C; heating the reaction kettle to 90 DEG C, and then dripping the rest 80 to 70 percent of mixed monomer aqueous solution and the rest 90 to 80 percent of ammonium persulfate initiator aqueous solution into the reaction kettle at the same time; and dripping the aqueous solution for 3 to 5 hours, then heating the reaction kettle to between 96 and 98 DEG C, preserving the heat for 1 hour, and cooling the reaction kettle to below 50 DEG C to obtain the polyhydroxy auxiliary agent for fly ash fiber papermaking. When the polyhydroxy auxiliary agent prepared by the method is applied to fly ash fiber pulp, the polyhydroxy auxiliary agent can improve the dispersion property of the fly ash fiber in the water, enhance the binding force between the fly ash fiber and wood pulp fiber or straw pulp fiber, and improve the strength of the fly ash fiber paper.