Abstract: Acid-catalyzed isomerization of β-phellandrene for the preparation of α-terpinene and relative reaction process were investigated. The results indicated that trimethylsilyl bromide(TMSBr) showed excellent catalytic activity on the isomerization β-phellandrene owing to its moderate acidity. However, its catalytic effects on other monoterpenes such as α-pinene, β-pinene, 3-carene, limonene and terpinene were limited. The auxiliary agent of 30%(mass fraction) H₂O₂ greatly improved the reaction efficiency owing to its three characteristics including converting TMSBr to H⁺ to catalyze the isomerization reaction due to the existence of water, dispersing H⁺ to reduce the occurrence of side reactions, and assisting the reaction by its own acidity. The optimal conditions for the preparation of α-terpinene from the isomerization of β-phellandrene were β-phellandrene of 0.1 mol, TMSBr of 0.002 mol, 30% H₂O₂ of 0.1 mol, temperature of 100 ℃, time of 6 h, and the raw material conversion and product selectivity were 88.4% and 94.0%, respectively.