The present invention relates to a method of preparing N-substituted maleimide in high purity and high yield, wherein, unlike a conventional method of preparing N-substituted maleimide after preparing N-substituted maleamic acid, an organic solvent, an acid catalyst, a dehydration co-catalyst and a stabilizer are added into a reactor without separately preparing N-substituted maleamic acid, and then a primary amine is added into the reactor to form an amine salt, maleic anhydride is added into the reactor, and the resulting mixture is subjected to a dehydration-cyclization reaction, wherein water produced during the reaction is azeotropically distilled with the organic solvent so as to effectively suppress formation of a maleimide polymer. Particularly, the invention simplifies the process, has high yield, minimizes side reactions, eliminates the introduction of expensive raw materials and the complicated treatment of the reaction solution and significantly shortens the time over which the raw materials and maleic anhydride are added.