The invention relates to a preparation method of a highly branched polyvinyl alcohol-acrylamide graft copolymer. After dissolving the polyvinyl alcohol in water, blowing nitrogen gas, lowering the temperature, adding a catalyst, reacting for 10-30 minutes, and then adding a monomer mixture solution dropwise , control the dropping time for 30-90 minutes, during which the reaction temperature is controlled between 65°C-95°C, after dropping, control the temperature between 85°C-95°C for 20-30 minutes, add catalyst, and continue at 80°C React at ‑95°C for 15-25 minutes, add catalyst, take samples during the reaction to measure the viscosity, after the viscosity reaches 1500-15000 mPa.s at 25°C, add a terminator to stop the reaction, adjust the solid content, and the product is ready. The graft copolymer of the present invention has high molecular weight, many branched chains, low viscosity, less gel, good retention, and strong anti-interference ability of impurities. As a dry strength agent for papermaking, it can also Satisfactory enhancement effect is achieved.