The invention discloses a preparation method of a cellulose-N-vinyl pyrrolidone radiation grafting copolymer. The preparation method comprises the following steps: step (1), adding cellulose into de-ionized water and swelling, and then adding N-vinyl pyrrolidone, wherein the mass ratio of the cellulose to the de-ionized water to the N-vinyl pyrrolidone is (11 to 14) to (14 to 17) to (3 to 4); step (2), carrying out oscillation treatment, wherein the oscillation frequency of a first phase is 50 times per minute to 70 times per minute and the oscillation time is 20min to 30min, and the oscillation frequency of a second phase is 30 times per minute to 40 times per minute and the oscillation time is 10min to 15min; step (3), irradiating, wherein an accumulated irradiation dosage is 6kGy to 10kGy and the irradiation dosage rate is 0.8kGy/h; and step (4), keeping the heat at a temperature ranging from 80 DEG C to 90 DEG C for 15min to 20min, and carrying out phase type oscillation treatment at the same time. By adopting the preparation method, the grafting reaction is uniformly carried out, and the adsorption performance of the grafting copolymer on Pb<2+> is improved.