The invention is aimed at providing a synthesis method for poly-N-vinylcaprolactam, so as to solve the problems of long time and toxic solvent in the present polymerization process. The method comprises the following steps of: A, reaction: performing reduced pressure distillation on N-vinylcaprolactam, then adding the N-vinylcaprolactam, an initiator azodiisobutyl imidazoline hydrochloride and a solvent in a three-neck flask, charging nitrogen to protect and starting to stir, and heating to 65-80 DEG C, wherein the solvent is one of absolute ethyl alcohol and water, and the usage amount of the initiator is 0.1-0.5 folds of the mass of the N-vinylcaprolactam; and B, post-treatment: steaming away the solvent at 50 DEG C and under a reduced pressure of 0.08 MPa to obtain a colourless sticky liquid, adding tetrahydrofuran to dissolve, and separating out by n-hexane, standing, removing the supernatant, adding ethyl ether to precipitate, filtering, and drying at 30 DEG C to obtain the product poly-N-vinylcaprolactam. The synthesis method provided by the invention eliminates the harm on environment by the utilization of toxic solvent; and the average molecular weight detected by GPC (gel permeation chromatography) is greater than 10000-30000.