The reactive oxygen metabolism under different salt levels and the alleviative effects of exogenous abscisic acid (ABA) on plants against salt stress were assessed in Platycladus orientalis seedlings treated with different concentrations of NaCl and ABA. P. orientalis seedlings were cultured by 1/4 Hoagland solution with different levels of NaCl (100, 200, 300 and 400 mmol·L^-1), and 1/4 Hoagland solution was used as control. Furthermore, seedlings exposed to 300 μmol·L^-1 NaCl were treated with different concentrations（0, 0.5, 1, 10, 100 and 200 μmol·L^-1） of ABA, and physiological indexes and expression levels of genes related to reactive oxygen metabolism were studied. The results showed that: (1) With the increase of NaCl level, the contents of hydrogen peroxide (H₂O₂), malondialdehyde (MDA), glutathione (GSH) and proline and activities of antioxidant enzymes (SOD, POD, CAT) in P. orientalis leaves increased, while soluble protein content decreased. (2) Salinitytreated plants exposed to ABA at 1 and 10 μmol·L^-1 for 48 h significantly enhanced the activities of SOD, POD and CAT, and increased the contents of GSH, proline and soluble protein, accompanied by decreased accumulation of H₂O₂ and MDA. (3) NaCl stress caused the variation of the expression levels of genes related to reactive oxygen metabolism in P. orientalis, and the presence of 10 μmol·L^-1 ABA maximally induced expression levels of Cu/ZnSOD, CAT, GR, APX, MDAR and GST genes in 300mmol·L^-1 NaCltreated seedlings. These results suggested that exogenous ABA could promote antioxidant enzyme activities, increase osmotic adjustment ability and GSH content, decrease the accumulation of MDA and H₂O₂ in leaves of P. orientalis under salt stress, thereby effectively reduce the damage of reactive oxygen species to P. orientalis leaves and enhance its salt resistance.