Abstract: The objective of these experiments was to determine the effects of exogenous nitric oxide (NO) on the antioxidant activity and photosynthetic characteristics of Prunella vulgaris seedlings under salt stress. Seedlings of P. vulgaris under salt stress (70 mmol/L NaCl) were treated with sodium nitroprusside (SNP, an exogenous NO donor) at different concentrations (0.01, 0.05, 0.10 and 0.50 mmol/L) and their antioxidant enzyme activity, photosynthetic pigment contents, gas exchange, and chlorophyll fluorescence characteristics were analyzed. The results showed that 0.05-0.10 mmol/L SNP alleviated symptoms of NaCl stress in P. vulgaris seedlings. The 0.10 mmol/L SNP treatment produced the best results. The seedlings in this treatment showed significantly increased (P<0.05) activities of peroxidase and catalase, decreased malondialdehyde content and electric conductivity, significantly increased (P<0.05) chlorophyll content, net photosynthetic rate, stomatal conductance, and transpiration rate, and decreased intercellular CO₂ concentration. Analyses of chlorophyll fluorescence data showed that the 0.10 mmol/L SNP treatment resulted in a significant decrease (P<0.05) in initial fluorescence and non-photochemical quenching, and increased maximum fluorescence, photosystem Ⅱ (PSⅡ) potential fluorescence efficiency, PSⅡ maximum fluorescence efficiency, and PSⅡ excitation capture efficiency in leaves of salt-stressed P. vulgaris seedlings. These results showed that application of SNP at certain concentrations stabilized membranes by increasing the activities of antioxidant enzymes, and improved photochemical efficiency. These changes alleviated photoinhibition damage resulting from salt damage to the PSⅡ center, and enhanced the photosynthetic capacity to increase the salt resistance of P. vulgaris seedlings. The 0.10 mmol/L SNP treatment had the greatest effect on improving salt tolerance of P. vulgaris seedlings under these experimental conditions.