NOVELTY - Oleic acid and methanol are provided to the reactor, catalyst is added and methanol is recycled, to obtain methyl oleate. The obtained oleate is decompressed and rectified, methyl oleate and solvent are provided to the reactor and ozone is filled, to obtain methyl oleate ozonide. The obtained methyl oleate ozonide is provided to the reactor, stirred, and sodium carbonate solution is added, to obtain azelaic semialdehyde methyl ester crude product. The obtained product is decompressed and rectified, to methyl octanoate product.    USE - Preparation of methyl octanoate used in pharmaceutical field.    ADVANTAGE - The methyl octanoate is efficiently and economically prepared with high yield without polluting the environment, by simple method.    DETAILED DESCRIPTION - Oleic acid and methanol are provided to the reactor, then catalyst is added, temperature is increased at 72 degrees C to reflow for 3-7 hours and methanol is recycled to normal pressure, to obtain methyl oleate. The obtained oleate is decompressed and rectified, methyl oleate and solvent are provided to the reactor and ozone reaction is filled to room temperature, to obtain methyl oleate ozonide. The obtained methyl oleate ozonide is provided to the reactor and reducing agent is added to room temperature, temperature is raised at 50-90 degrees C, continuously stirred and reacted for 20-60 minutes, and then sodium carbonate solution is added, to obtain aldehyde and methyl octanoate crude product. The obtained crude product is decompressed and rectified, to obtain methyl octanoate product. The molar ratio of oleic acid and methanol is 1:3-9. The addition amount of catalyst is 1-10 %mass with respect to total amount of oleic acid. The catalyst is toluenesulfonic acid or concentrated sulfuric acid. The mass ratio of methyl oleate and solvent is 1:2-8. The solvent is mixture of water and toluene, or mixture of water and dimethyl benzene. The reducing agent is sodium metabisulfite or water solution of sodium thiosulfate.