The invention discloses a synthesis method of N-phenylmaleimide (N-PMI). Under the action of a catalyst, the N-phenylmaleimide is directly synthesized by aniline and maleic anhydride in an organic solvent; the mass ratio of the organic solvent to maleic anhydride is (0.2-0.45) to 1; the mass ratio of aniline to maleic anhydride is (0.8-0.9) to 1; the reaction temperature is 135 DEG C-140 DEG C; and the reaction time is 0.8h-1h. By adopting the method provided by the invention, wastewater can be effectively reduced; and after the reaction is finished, the amount of acid remained in an N-PMI solution is few, so that the amount of water and alkali, which are used for water washing and neutralization in a post-treatment process, can be reduced. The synthesis method is a environment-friendly process route.