The invention relates to a preparation method of 3-methyl phenylhydrazine oxalate. The preparation method comprises the following steps: diazotization, reduction, purification and salt formation. In the diazotization and reduction steps, reaction liquid is kept with high acidity by means of concentrated hydrochloric acid, so that the reaction is smoothly and fully carried out. In the reduction step, zinc powder-concentrated hydrochloric acid is taken as a reducing agent to replace sodium thiosulfate, sodium hydrogen sulfite, stannous chloride-hydrochloric and the like, the reducing property isgood, the yield is high, the reaction time is shortened, impurities such as zinc hydroxide generated by the reaction are conveniently removed, and the product is few in impurity and high in purity. In the salt formation step, acetone is used to wash, so that the purity of the product is improved, and the appearance of the product is ensured. According to the preparation method, the process is stable and reliable, the operation is easy, the product purity is high (the content of the product detected by high performance liquid chromatography is greater than or equal to 99%), the yield is greater than or equal to 42%, and the demand on 3-methyl phenylhydrazine oxalate in the market is fully met.