Abstract: For the oil samples before and after fixed bed hydrogenation reaction at different space speeds, transformation behavior of iron and calcium compounds were studied. The oil samples were decomposed with acid and then extracted with aqueous alcoholic solution of sodium hydroxide. The corresponding petroleum acids derived from petroleum acid salts were obtained. The structure of petroleum acid was characterized by infrared spectrometry, element analysis, hydrogen nuclear magnetic spectrum, nuclear magnetic carbon and high resolution mass spectrometry. In order to study the transformation of petroleum acid in thermal reaction, the obtained petroleum acid was characterized by thermogravimetric mass spectrometry. The transformation process of petroleum acid in thermal reaction was analyzed, and then the transformation process of iron and calcium compounds in thermal reaction was obtained. The results show that the main components of oil acid in residue before and after the hydrogenation reaction are naphthenic acid existing as the form of polymer. Before and after the thermal modification process, petroleum acid is decomposed obviously, and the main products are CO₂, C₃H₈ and other substances. With the increase of reaction depth, the degree of decarboxylation and chain breaking reaction of petroleum acid increases, which further makes petroleum acid decomposition.