In order to study the effects of exogenous melatonin and silicon on salt tolerance of celery seedling and clarify the mechanism,a celery cultivar—Huanghou was selected as experimental material,the effects of single and compound exogenous melatonin and silicon on the plant growth,photosynthesis,antioxidant system and osmotic regulation system of celery seedling under salt stress were investigated.The results showed that under salt stress condition,the growth and photosynthesis of celery seedling were significantly inhibited,the activities of SOD,POD,CAT and APX were significantly enhanced,and the contents of H₂O₂,MDA,soluble sugar,soluble protein and proline were significantly increased.Single or combined application of exogenous melatonin and silicon significantly improved the growth of celery seedling,significantly reduced MDA content and H₂O₂ content,significantly raised Pn,Gs and Tr values,significantly enhanced the activities of antioxidant enzymes(POD,SOD,CAT and APX) of celery seedling leaves under salt stress and the best effect was from the combination of exogenous melatonin and silicon.Under the salt stress,compared with salt stress,the plant height,stem diameter,leaf area,dry weight,Pn,Gs,Tr,SOD,POD,CAT and APX activities,soluble sugar,soluble protein and proline contents of celery seedlings treated with the combination of exogenous melatonin and silicon increased by 22.24%,24.33%,46.74%,14.89%,59.30%,82.40%,54.11%,33.18%,40.35%,30.95%,66.43%,48.64%,58.94%and 44.09% respectively,and the MDA content and H₂O₂content were decreased by 42.94% and29.93% respectively.These results suggest that single or combined application of exogenous melatonin and silicon effectively alleviate the salinity damage and improve the resistance of celery seedlings under salt stress by improving photosynthesis,active oxygen scavenging capacity and osmotic balance regulation capacity of celery seedling,and a synergistic effect is observed after applying both exogenous melatonin and silicon.