Petroleum coke can be divided into delayed coke and fluid coke according to their coking processes. These two kinds of petroleum coke, although with similar elemental compositions, normally have significant differences in their physicochemical structures. In this study, the carbon microcrystal structures of these two petroleum cokes were first characterized using X-ray diffraction and Raman analysis. The results showed that the carbon structure of fluid coke was more ordered and its degree of graphitization was higher. Gasification of these two petroleum cokes in a thermogravimetric analyzer proved that the difference of their reactivity was consistent with the changing trend of their carbon structures. Moreover, the Fourier transform infrared spectroscopy (FTIR) characterization was carried out and justified that the large removal of O-containing chemical bonds in fluid coke was another major reason for its low reactivity. On this basis, blast furnace (BF) slag was used to catalyze the gasification reaction of these two petroleum cokes. The results showed that BF slag had significant catalytic effects on the gasification of petroleum coke. Via the FTIR characterization of samples with different conversion ratios, we found that the BF slag could destroy the C=C aromatic structure in petroleum coke and meanwhile promote the formation of active C(O) compounds on the surface of delayed/fluid coke, in return, increasing the gasification rate.