The invention relates to a purifying method of 4-nitro-3,5-dimethylpyridine-N-oxide, and the method comprises the following steps: adopting 3,5-dimethylpyridine-N-oxide to perform nitration reaction and obtain 4-nitro-3,5-dimethylpyridine-N-oxide reaction solution, adding alkali in the reaction solution to adjust the pH value to 4-6 so as to precipitate solid, filtrating, using organic solvent towash filter cake, performing extraction to filtrate, performing reduced pressure distillation to organic phase after delamination to obtain raw 4-nitro-3,5-dimethylpyridine-N-oxide, washing the raw product with absolute methanol, filtrating and finally using petroleum ether for recrystallization. The product obtained in the invention has high purity, and the purification method is simple and is applicable to industrial application.