Abstract: By using solution culture method, this paper studied the effects of exogenous nitric oxide (NO) on the antioxidants and antioxidases in the ascorbateglutathione (AsA-GSH) cycle in tomato (Lycopersicon esculentum Mill.) seedling roots under copper stress. Exogenous NO could affect the metabolic cycle of AsA-GSH in tomato  roots under copper stress. Applying appropriate amount of exogenous NO increased the AsA and GSH contents, AsA/DHA and GSH/GSSG ratios, and decreased the DHA and GSSG contents in tomato  roots under copper stress. With the addition of 100 μmol·L^-1 of BSO, exogenous NO increased the AsA content, AsA/DHA ratio, and the AAO, MDHAR, and DHAR activities, and decreased the DHA, GSH, and GSSG contents and the APX and GR activities. When 250 μmol·L^-1 of BSO was added, exogenous NO increased the contents of AsA, GSH, and GSSG, AsA/DHA ratio, and the activities of APX and GR, and decreased the DHA content and the AAO, DHAR and MDHAR activities. It was suggested that exogenous NO could affect the metabolic cycle of AsA-GSH in tomato  roots under copper stress, and mitigate the damage of copper stress to tomato  roots via regulating the AsA/DHA and GSH/GSSG ratios to alleviate oxidative stress.