Abstract: Naphthenic acid in the distillates of 200—350 ℃ and 350—450 ℃ in Shengli high acid crude oil were separated and purified through alkaline-ethanol extraction. The structures and compositions of naphthenic acids were confirmed by means of element analysis method, steam osmosis pressure，hydrogen nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared spectroscopy techniques. The corrosion of A20 carbon steel in the two distillates was conducted at different temperatures. The results show that naphthenic acid is monocarboxylic, and the average molecular formulas of naphthenic acid in the distillates of 200—350 ℃ and 350—450 ℃ are C15.86H27.51O2.06 and C32.86H54.77O2.29, respectively. The corrosion ability of naphthenic acid is not only related to acid content, but also closely related with its structure. As the fraction becomes heavier, the branching degree of petroleum acid molecule increases from 0.197 to 0.292, and the electron-withdrawing property of the group connected with carboxyl groups in acid reduces, and the acidity decreases, resulting in a lower corrosion rate of carbon steel. At the same time, a larger space steric hindrance in branched acid molecules would lower the corrosion ability of naphthenic acid. The corrosion rate is increased as temperatures go up and the corrosion ability of the small molecular naphthenic acid is stronger.