Galaxolide (HHCB), an important emerging contaminant, has attracted great environmental concern owing to its widespread occurrence and potential toxicity. In this study, the detailed multichannel mechanism of OH radical-initiated atmospheric degradation reactions of HHCB has been investigated by employing density functional theory (DFT). The reactants, transition states, intermediates, and products were optimized at the MPWB1K/6-31+G(d,p) level, and single-point energies were further refined at the MPWB1K/6-311+G(3df,2p) level of theory. The canonical variational transition-state (CVT) theory combined with the small curvature tunneling (SCT) was performed to evaluate the Arrhenius expressions and rate constants of key elementary reactions over a suitable range of 180-370 K. The thermodynamic and kinetic calculation results show that OH addition and hydrogen abstraction reactions are competitive pathways for HHCB. The dominant products in the presence of O-2/NO are epoxide, dialdehyde, alcohol ketone, cyclolactone compounds, and HO2 radicals. At 298 K, the total rate constant of OH-initiated degradation of HHCB is 2.71 X 10(-11) cm(3) molecule(-1) s(-1). The atmospheric lifetime of HHCB determined by OH-initiated reactions is 10.09 h, which is in favor of the phenomenon of medium-range transport for HHCB in the atmosphere.