Abstract: Iso-butyl, iso-octyl mono-substituted or double-substituted mixed alkylated diphenylamine antioxidants were synthesized in one step by used diphenylamine and diisobutylene as raw materials under the action of acidic catalysts. By optimizing the process conditions such as catalyst dosage, raw material ratio, reaction temperature, and reaction pressure, a higher mass fraction of butyl/octyl-substituted diphenylamine antioxidant was obtained, and the mass fraction of diphenylamine was reduced to below 0.50%. The antioxidant performance of the amplified synthesized product was evaluated by rotating bomb oxidation test (RPVOT) and pressure differential scanning calorimetry (PDSC). The results showed that the synthesized high content butyl/octyl substituted diphenylamine had excellent antioxidant effects, superior to commercially available imported products, and could meet the low-temperature flowability requirements of marine turbine oil.