The invention provides a synthetic method for milbemycin oxime. The method comprises the following steps that 1, an oxidation reaction is performed, wherein milbemycins is used as raw materials, pypocholoride or chlorite is used as an oxidizing agent, piperidines free radical of nitroxide is used as a catalyst, a halide is used as a promoter, all the materials are subjected to the reaction in a dichloromethane solvent at the temperature being minus 5-15 DEG C for 0.5-4 h, and postprocessing is performed on a reaction product to obtain an intermediate product of milbemycin ketone; 2, an oximation reaction is performed, wherein methyl alcohol and 1,4-dioxane are used as a reactive solvent, hydroxylamine hydrochloride is used as an oximation reagent, the reaction is performed at the temperature being 25-35 DEG C for 10-16 h, and postprocessing and purification are performed to obtain milbemycin oxime. According to the method, industrial production of the milbemycin oxime is achieved in China for the first time, and the yield of the obtained product is higher than that of similar products both at home and abroad. Due to the fact that the pypocholoride is used as the oxidizing agent, pyridine nitroxide free radical is used as a catalyst, conditions are mild, few side reactions exist, the yield is high, and the cost is low.