1-Chloro-2,4,5-trimethyl-benzene was synthesized by one-step method, unsym- trimethylbenzene as the raw material, while HCl-H ₂O ₂-CH ₃COOH as the catalytic activity system. By the orthorhombic experiment, the influenced factors are considered and the optimum conditions are confirmed. The experiment was carried under 60℃ for 2 h, while the mole ratio of H ₂O ₂ and unsym- trimethylbenzene is 4.5, HCl and unsym- trimethylbenzene is 4.0. The average yield was 82.8%, and the conversion rate was 98.4% while the fineness 99% under those conditions.