Disclosed is a simple synthesis method of lactam derivatives, comprising: with formamide functioning as both an amine source and a hydrogen source (hydrolyzed to produce formic acid), carrying out a cycloamination reaction on a raw material keto acid in the absence of a solvent or a catalyst to simply synthesize a lactam derivative. Compared with previous reports, the present disclosure has the following advantages: the time required for the reaction is greatly shortened, the selectivity is remarkably improved, a conversion rate of a keto acid derivative is greater than 99%, and the yield of the lactam derivative can reach 70% to 94%.