The invention discloses a method for preparing a polymer flocculant with materials recovered from waste fluid generated by pulping and papermaking, wherein, the method comprises the following steps: inletting Nitrogen into a three-neck flask which is in a water bath kettle at temperature ranging from 20 DEG C to 70 DEG C so as to drive away air, adding a prepared acrylamide aqueous solution with a mass concentration of 8.0-30.0 percent into the three-neck flask, adding an initiating agent into the acrylamide aqueous solution while stirring, dripping a lignosulfonate solution into the three-neck flask when the solution is viscous and reacting for 2-5 hours at the temperature ranging from 20 DEG C to 70 DEG C; the product obtained is a graft copolymer of the lignosulfonate and the acrylamide. The lignosulfonate solution is a lignosulfonate aqueous solution with a concentration of 13.0-30.0 percent, wherein, the lignosulfonate is flocculated and subsided from pulping waste fluid; a sulfuric acid solution with a concentration of 4-10 percent is used for adjusting the pH value of the lignosulfonate solution to 5-12. The method reasonably utilizes the papermaking water fluid, which is beneficial to the environmental protection; that the flocculant treats waste water (including printing and dyeing sewage, domestic sewage, scouring sewage, and the like), with equivalent dosage, facilitates solid-liquid separation and has a better treatment effect than that of conventional flocculants as well as a low price.