Butylated hydroxyanisole (BHA) is one of important phenolic antioxidants and its fate in the environment has attracted much attention in recent years. In this study, the initial reactions of BHA with OH radicals, including 8 abstraction reactions and 6 addition reactions, were calculated. The lowest energy barrier of 3.20 kcal mol⁻¹ was found from the abstraction reaction on phenolic hydroxyl group. The reaction barriers of addition paths are in the range of 5.48–9.28 kcal mol⁻¹, which are lower than those of the abstraction paths. The reaction rate constants were calculated by using transition state theory, and the rate constants are 8.12 × 10⁷ M⁻¹ s⁻¹ and 4.76 × 10⁷ M⁻¹ s⁻¹ for the H-abstraction and OH-addition reactions, respectively. Through the calculation of the subsequent reactions of the abs-H₀-TS₁ and add-C₄-M₁ it was found that BHA would be further transformed into 2-tert-Butyl-1,4-benzoquinone (TBQ), tert-butylhydroquinone (TBHQ) etc. in the aqueous phase, and the eco-toxicities of these transformed products of BHA in the aqueous phase were significantly increased comparing with that of the BHA and they are toxic to aquatic organism.