Activated carbon fiber (ACF) was used as a green catalyst to activate persulfate (PS) for oxidative decolorization of azo dye. ACF demonstrated a higher activity than activated carbon (AC) to activate PS to decolorize Orange G (OG). The decolorization efficiency of OG increased as ACF loading, PS dosage, and temperature increased. OG decolorization followed a pseudo first-order kinetics, and the activation energy was 40.902 kJ/mol. pH had no apparent effect on OG decolorization. Radical quenching experiments with various radical scavengers (e.g., alcohols, phenol) showed that radical-induced decolorization of OG took place on the surface of ACF, and both SO4·− and HO· were responsible for OG decolorization. The impact of inorganic salts was also evaluated because they are important compositions of dye wastewater. Cl− and SO42− exhibited a promoting effect on OG decolorization, and the accelerating rate increased with elevating dosage of ions. Addition of Cl− and SO42− could increase the adsorption of OG on ACF surface, thus favorable for OG decolorization caused by the surface-bound SO4·− and HO·. Conversely, HCO3− and humic acid (HA) slightly inhibited OG decolorization. The azo band and naphthalene ring on OG were remarkably destructed to other intermediates and finally mineralized to CO2 and H2O.