The present invention relates to a synthesis method of ultra low molecular weight homopolymerized N-vinyl butyrolactam with ultra low residual monomer content, which comprises: using N-vinyl butyrolactam monomers and water with the mass ratio of 3:17˜8:12, then based on the N-vinyl butyrolactam monomers, using 0.5%˜5.0% by weight of initiators, 0.1%˜5.0% by weight of a catalyst, and 0.1%˜10% by weight of a molecular weight regulator; in the presence of an inert gas, adding the above mentioned raw materials in batches, adjusting the pH of the reaction system to 7.0-8.0 with an activator, reacting at the polymerization temperature of 60-85° C.; finally adding a peroxide and keeping the temperature for 2 hours, to obtain an aqueous solution of ultra low molecular weight homopolymerized N-vinyl butyrolactam, drying the aqueous solution to obtain a powdery ultra low molecular weight homopolymerized N-vinyl butyrolactam with ultra low residual monomer content, whose K value is 12-17, whose molecular weight is 2000-15000, and whose residual monomer content is less than 10 ppm, so the present invention is designed skillfully and simple in preparation, the residual monomer content of the ultra low molecular weight homopolymerized N-vinyl butyrolactam with ultra low residual monomer content prepared is less than 10 ppm, and the K value is 12-17, therefore the present invention is suitable for large-scale popularization.