•Alkaline stress caused growth inhibition of cucumber seedlings.•Application of salicylic acid (SA) matained ion balance under alkaline stress.•Application of SA improved photosynthetic capacity under alkaline stress.•Application of SA alleviated oxidative damage caused by alkaline stress.•Application of SA improved alkaline-tolerance of cucumber seedlings. Although various works have been done in alleviating effects of exogenous salicylic acid (SA) on abiotic stress in plants, it was not reported whether exogenous SA had effects on alkaline-tolerance of plants. In the present study, we found that the effect of exogenous SA depended on the concentration and 75 μM SA showed the best remission effect on cucumber plants under alkaline stress. The effect of 75 μM SA on detailed change of photosynthetic capacity, reactive oxygen metabolism and ionic homeostasis in alkaline-treated cucumber plants were further investigated. Alkaline stress significantly caused ion imbalance, decreased photosynthetic pigment content, inhibited PSII activity and net photosynthetic rate. With prolonging stress time, electrolyte leakage, MDA and H2O2 contents were significantly increased in cucumber leaves, while antioxidative enzymes activity and ascorbate-glutathione cycle increased first and then decreased. Application of 75 μM SA reduced Na+ accumulation, maintained ionic homeostasis and normal operation of photosystem, enhanced the reactive oxygen species (ROS) scavenging system, reduced oxidative damage, and alleviated lipid peroxidation, thereby improved the alkaline-tolerance of cucumber plants.