The invention provides a method for preparing 4-(3-methoxy propoxy)-2,3-dimethyl pyridine-N-oxide, comprising the following steps: mixing 2,3-dimethyl pyridine-N-oxide (a), a bromine source and a solvent, carrying out heating reaction to obtain 2,3-dimethyl-4-bromo pyridine-N-oxide; mixing the 2,3-dimethyl-4-bromo pyridine-N-oxide with 3-methoxy propanol, and carrying out heating reaction to obtain a target compound. Compared with the method of nitrifying the 2,3-dimethyl pyridine-N-oxide (a) and then directly subjecting the 2,3-dimethyl pyridine-N-oxide (a) to react with the 3-methoxy propanol and potassium carbonate to obtain the target compound in the prior art, the method provided by the invention has the advantages that: the nitration is omitted, the use and consumption of the mixed nitric acid and sulfuric acid are avoided, and the pollution of a large amount of waste acid to the environment is reduced. In addition, the reaction activity of the bromide is higher than that of the corresponding chloride, and thus the 3-methoxy propoxy is easier to react in one step with a short reaction time. Moreover, the bromine containing wastewater generated from the reaction can be reutilized.