Abstract: A complex dimeric acid was synthesized by Diels-Alder reaction using industrial fatty acid and rosin as raw material. The contents of monomer, dimer and trimer were detected by gel permeation chromatography(GPC). Effects of different reaction conditions on the composition and properties of product were discussed. The result showed that using the amount of the self-made catalyst 1% in terms of mass of the system, mass fraction of rosin 5%, reaction time 5 h, reaction temperature 240℃ and reaction pressure 0.5 MPa, 51.03% mass fraction of dimeric acid can be obtained in the product. The viscosity of product is 320 mPa·s at 40℃ and the acid value of the product is 193.9 mg/g. The product was characterized by 1H NMR, which showed that addition reaction had taken place.