Selenium (Se) stress usually hinders plant growth by inducing cell injury. Cinnamaldehyde (CA) is a kind of natural chemical with antioxidative properties. In this study, we used physiological, biochemical, and in situ fluorescence detection approaches to study the role of CA in the alleviation of Seinduced phytotoxicity in the roots of nonheading Chinese cabbage (Brassica rapa) seedlings, which might involve in the regulation of polyamine oxidasehydrogen peroxide (PAOH₂O₂) system. Basically, we obtained the following results. (1) Se stress significantly inhibited root growth in a dosedependent manner. The addition of CA remarkably alleviated the root growth inhibition caused by Se stress. (2) CA alleviated root oxidative damage and cell death induced by Se stress. (3) Se stress led to a significant increase in PAO activity and H₂O₂ level. The addition of CA significantly decreased PAO activity and H₂O₂ level. (4) Among the six BrPAOs family genes (BrPAO16) in B. rapa，Se stress significantly upregulated the expression levels of BrPAO3, BrPAO5 and BrPAO6, an effect that was reversed by the addition of CA. These results suggested that CA was able to alleviate Seinduced phytotoxicity by repressing PAOH₂O₂ system. The results provide new evidence for exogenous regulation of Se tolerance in crops.