The naphthenic acid corrosion (NAC) behaviors of SA210C and A335-P5 steel at different temperatures and flushing angles were investigated by a high-temperature high-flow-rate naphthenic acid corrosion simulation device. It was found that temperature and flushing angle could greatly influence the corrosion rate of SA210C and A335-P5, and temperature was the most important factor. Metallographic observations of SA210C and A335-P5 showed that perlites in ferrite steels would be dissolved preferentially when NAC occurred, and the ferrites would not be corroded remarkably before the perlites were dissolved completely. When the ferrites were corroded, the corrosion proceeded from grain boundary to inner grain. The subsurface would not be corroded before the perlites and ferrites vanished completely on the surface.