Abstract: This study investigated the effect of exogenous salicylic acid (SA) on the physiology, biochemistry, nitrogen metabolism and secondary metabolism of Atropa belladonna seedlings under NaCl stress. We cultivated A. belladonna seedlings using soilless culture, treated with 100 mmol/L NaCl; 0.75 mmol/L SA was also sprayed onto the surface of the leaves. After application of SA, chlorophyll content increased and maximal photochemical efficiency (F_v/F_m), potential activity of PSⅡ (F_v/F_o), the efficiency of excitation energy capture by open PSⅡ centers (F_v'/F_m'), photosynthetic electron transport rate (ETR), actual PSⅡ photochemical efficiency (ΦPSⅡ) and photochemical quenching coefficient (qP) were all greatly enhanced, while initial fluorescence (F_o) and non-photochemical quenching coefficient (qN) significantly decreased. The activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) dramatically increased, while malondialdehyde (MDA) content decreased dramatically. Soluble protein content notably increased, while the contents of proline and soluble sugar decreased. The NO3- content increased, while the NH4+ content decreased. The activities of glutamine synthetase (GS) and glutamate synthetase (GOGAT) increased, while glutamate dehydrogenase (GDH) activity decreased. The secondary metabolites, scopolamine and hyoscyamine, were increased significantly. The study indicated that exogenous SA could enhance photosynthesis in A. belladonna, adjust organic osmotic content, increase antioxidant enzyme activities and reduce the degree of membrane lipid peroxidation, alleviating the damage of NaCl stress. The recovery or enhancement of the enzyme activities of nitrogen metabolism promoted the synthesis and accumulation of secondary metabolites directly or indirectly by providing more precursors and energy for the synthesis of tropane alkaloids.