【Objective】 In relatively enclosed greenhouses, soil salts tend to accumulate gradually in topsoil duo to the special greenhouse environment of no rain water leaching, high temperature and resultant high evaporation, thus leading to serious secondary salinization of the greenhouse soil. According to relevant reports, the high level of Ca(NO₃)₂ accumulation was one of the main causes of high soil salinity in greenhouses. Nitric oxide (NO) is a kind of micromolecule active material that generally exists inside plants. As a plant hormone and signaling molecule, NO extensively participates in regulating of plants’ responses to various adversity stress. The aim of this paper is to investigate roles of exogenous NO in regulating the antioxidant system of plants under Ca(NO₃)₂ stress. 【Method】A hydroponic experiment was conducted to investigate effects of foliar spray of sodium nitroprusside (SNP) as exogenous NO donor on growth of tomato (‘Qin Feng Bao Guan’) seedlings, and photosynthetic, reactive oxygen, anti-oxidase activities, and ascorbate-glutathione cycle in their leaves under the stress of 80 mmol·L^-1 Ca(NO₃)₂. 【Result】In the leaves of the tomato seedlings under Ca(NO₃)₂ stress, foliar spray SNP significantly lowered O₂^·- production rate, H₂O₂, malondialdehyde, dehydroascorbic acid and oxidized glutathione in content and electrolyte leakage rate, but heightened or maintained superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase in activity; besides, it significantly increased the contents of ascorbic acid and glutathione and their reducing power. As a result, active oxygen damage of the leaves was effectively alleviated, and chlorophyll degradation and photosynthetic rate declining trend were effectively suppressed. 【Conclusion】 Exogenous NO treatment plays an important role in maintaining or improving the activity of antioxidant enzymes and promoting the operation of aseorbate-glutathione (AsA-GSH) cycle in tomato under the stress of Ca(NO₃)₂, thus relieving the plants of oxidative damage significantly, and improving their leaf membrane system in stability, their photosynthetic function, their tolerance to salt and eventually their growth.