Abstract: In the refineries processing the high acid crude oil, the vacuum towers of the crude oil units were usually subject to naphthenic corrosion in the high temperature area. Take the severe corrosion happened in the third vacuum stream oil system of one vacuum tower for example, the corrosion causes were analyzed from the aspects of oil quality, temperature, material, fluid’s physical condition, high-temperature corrosion inhibitor and so on. The results show that: due to the poor quality of the crude oil, naphthenic acid concentrated in the third vacuum stream oil system of the vacuum tower. In addition, Mo content of the materials didn’t meet the requirement and the flow rate of the medium in the tower was high, therefore the severe naphthenic corrosion happened in this area under the combined action of these factors. In the end, suggestions on mitigating the naphthenic corrosion in vacuum tower were proposed according to the corrosion causes.