Cucumber cultivar ‘Jinyan 4’ was subjected to suboptimal temperature treatment of 18/12 ℃ (day/night) in the growth chambers. A solution culture experiment was conducted to study the effect of exogenously spraying 5 mg·L^-1 chlorophylliniron solution on plant growth, the content of proline, soluble sugar, MDA and activity of peroxidase in the leaves of cucumber seedling under suboptimal temperature. Application of chlorophylliniron showed prominent effects on mitigating the stress of suboptimal temperature on growth of the cucumber seedlings, significantly increasing the plant height, leaf area, shoot dry mass, the contents of soluble sugar and proline and the activities of SOD, POD, CAT and APX. Exogenously spraying chlorophylliniron could promote the accumulation of proline and soluble sugar, raise the activities of antioxidant enzymes, decrease the membrane lipid peroxidation and improve the adaptability of cucumber seedlings under suboptimal temperature.