Abstract: In this experiment the Cucumis sativus L. variety ’Guifei rabbit leg’ was used as the test material to explore the effects of different concentrations of exogenous melatonin（MT） on Cucumis sativus L. seed germination and seedling growth under PP333 stress. The results showed that in the seed germination stress test, the exogenous MT soaking treatment of 100 mg/L inhibitting the cotyl length, root length and fresh weight at the germination stage of Cucumis sativus L. seeds could significantly relieve, while weakening the inhibitory effect of PP333 on seedling roots and hypocotyls; In the seedling stress test, the MT concentration of 100 mg/L could significantly promote the plant height and hypocotyl of seedlings coarse, leaf area and root length; Exogenous MT spraying with 100 mg/L significantly increased the soluble protein content, SOD activity and root activity of Cucumis sativus L. leaves, and significantly reduced the accumulation of harmful substances MDA and the relative content of proline and chlorophyll in plants. The study showed that the 100 mg/L MT could effectively alleviate the excessive inhibitory effect of PP333 stress on cucumber seed germination and seedling growth, improve the adaptability of cucumber seedlings, and ensure the normal plant of growth and development.