Analogue sulfur-containing precursors (ASCPs) were employed to prepare activated carbon (AC). Samples were characterized and analyzed by X-ray adsorption near-edge structure spectroscopy, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, ion chromatography, and N-2 physical sorption means. The results show that FeS2 in the precursor can react with the activation agent KOH to produce sulfur compounds such as K2SO4, K2S2O3, K2S, and thioether (R-S-R'), and reduce the actual KOH amount used for porosity development, hence decreasing specific surface area and pore volume of AC. This imposes an obviously negative effect on the capacitance and rate capability of the AC electrode as well as producing the byproduct Fe3O4. Consequently, FeS2 content in minerals as supercapacitor AC precursors should be limited to as low as possible in practical application. Furthermore, it is interesting to note that some of the inorganic FeS2 in the precursor can be converted into organic sulfide as a basic part of the resultant AC by undergoing a complex KOH activation process.