The invention relates to a flocculant preparation method. The flocculant preparation method comprises the steps of A, cleaning cellulose; B, immersing the cellulose into a solution with the PH of 7.5-8, conducting digestion for 1-2 h, and conducting cleaning 2-3 times; C, putting the cellulose in an environment of minus 5-minus 10 DEG C to be frozen; D, taking out the cellulose, putting the cellulose into an acid solution with the PH of 6.5-6.8, conducting soaking, and after the temperature goes back to 5-10 DEG C, taking out the cellulose to be cleaned 2-3 times; E, drying and crushing the cellulose into powder; F, putting the powder in water at the temperature of 0-4 DEG C; G, maintaining the temperature to be 0-4 DEG C, conducting stirring, and meanwhile slowly adding an alkaline reagent to adjust the PH to 7.3-7.5 to obtain a uniform and transparent solution; H, adding the solution into a vacuum sealing kettle, adding an oxidizing agent, and raising the temperature to 40-50 DEG C for a reaction of 2-3 h; I, rotatably evaporating the solution. The method has the advantages that a prepared flocculant is safe and environmentally friendly, inhibits bacteria, can be degraded, and has a good flocculation effect.