To investigate the mechanism of exogenous ABA alleviate the hyperhydricity of garlic plantlet in vitro, we studied the effects of exogenous abscisic acid (ABA), hydrogen peroxide (H₂O₂) and H₂O₂ +ABA on hyperhydricity, accumulation and localization of reactive oxygen species, and response of antioxidant system of plantlet in vitro with garlic variety ‘Ershuizao’. The results showed: (1) H₂O₂ aggravated hyperhydricity, the hyperhydric rate was the lowest under ABA treatment, and adding ABA into media with H₂O₂ alleviated hyperhydricity induced by H₂O₂. (2) O^-·₂ production rate and H₂O₂ content was the lowest under ABA treatment and the highest under H₂O₂ treatment. Adding ABA to the medium with H₂O₂ reduced the O^-·₂ production rate and H₂O₂ content. (3) The activities of catalase (CAT), peroxidase (POD) and ascorbate oxidase (APX) increased at the early stage during 0-8 day of H₂O₂ treatment, but then antioxidant enzyme activities of CAT, APX decreased and that of POD increased slowly during 8-16 day. SOD, CAT, POD, GR and APX activities increased linearly at 0-8 d under treatment of ABA and H₂O₂ + ABA. Activities of SOD, CAT, POD and APX under H₂O₂ + ABA treatment increased and were significantly higher than those of H₂O₂ treatment during 8-16th day. (4) The contents of ascorbic acid (AsA) and glutathione (GSH) increased at first and decreased later, were the highest under ABA treatment and the lowest under H₂O₂ treatment. (5) Tissue localization results showed that O^-·₂ and H₂O₂ were mainly produced in base of plantlets and tips of leaves. The accumulation of reactive oxygen species was the lowest under ABA treatment. (6) The content of malondialdehyde and membrane relative permeability under treatment of H₂O₂ + ABA was significant lower than that under H₂O₂ and control treatments. In summary, exogenous ABA can improve the antioxidant capacity of plantlets in vitro, reduce the accumulation and diffuse of reactive oxygen species, thereby inhibiting the occurrence of hyperhydricity.