The effect of sulfur and calcium impurities on coke reaction was investigated by simulating petroleum coke with low-impurity pitch coke and via impurity doping. Its mechanism has often been discussed via X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive spectrometry. The results show that sulfur has a strong catalytic effect on both the reactivity of coke in air and CO2 in the case with no other impurity interference. Its catalysis is probably realized by triggering a reaction system. However, during the carbon-air and carbon-CO2 reactions, an obvious mutual inhibition between the catalytic impurities of sulfur and those of calcium occurred when calcium was added. Inert CaSO4 and weak catalytic CaS are likely to be the key to producing this mutual inhibition.