A high-efficiency flocculant for heavy oil gasification carbon black waste liquid and its preparation method and application, which is characterized by the composition of the flocculant: polyacrylamide, EDTA-2Na, ammonium persulfate, phosphoric acid, formaldehyde, acetaldehyde and water. The method is to select polyacrylamide monomers, prepare monomer solutions, add EDTA-2Na, ammonium persulfate, phosphoric acid and other initiators, and react after filling nitrogen and driving oxygen. When the reactants are viscous, add formaldehyde to the viscous solution and acetaldehyde, and then add H3PO4 to catalyze, granulate, dry and pulverize the rubber block to obtain N-diphosphonoethyl-methacrylamide product. The invention can make the carbon black slurry of heavy oil gasification go through flocculation first, and then adopt the traditional filtration technology, so that the SS concentration of the filtered water can reach the first-class standard in GB13458-92, and at the same time recover high-quality and high-priced carbon black powder and realize low cost run.