C_22 -Monomethyl ester of dicarboxylic acid was synthesized from the methyl ester of cottonseed oil by converting non-conjugated methyl ester of linoleic acid to conjugated methyl ester of linoleic acid with fumaric acid via Diels-Alder reaction in the presence of catalyst. Orthogonal design was adopted to study the infleucnes of ratio of reaction mass, reaction temperature, amount of catalys and reaction time. Optimal process conditions were obtained in the presence of iodine as follow: tome ratio of fumaric acid to methyl ester of linoleic acid was 1:1.1; reaction temperature was 200～210℃ and reaction time 2h. The weight of iodine deed was 3% of that of methyl ester of cottonseed oil. The conversion of the methyl ester of linoleic acid was 98%. The process can be easily carried out in industry.