Abstract: To study the naphthenic acid distribution in refinery units, the samples from CDU，FCC，coking，hydrotreating and reforming units were collected. The acid number distribution and changes in each unit was obtained. The molecular structures of acids were characterized in some important units. It is found that high temperature corrosion occurs in the side cut No.2 and 3 of CDU, cut 1-2 of VGO as well as vacuum residues; There is a significant difference of acid distributions measured by true boiling point distillation and atmospheric and vacuum distillation; Compared with FCC feed，the acid amounts of each carbon number in FCC diesel and slurry are very low. The double-ring naphthenic acid is the main acid in above 3 kinds of fractions. Similarly，the acid amounts of each carbon number in coking diesel and CGO are also very low, but the main acid is single loop naphthenic acid and less decomposed. These results are important for acidic crude processing, anti-corrosion and product quality controlling.

Key words: acidic crude, acid distribution, acid characterization, corrosion