Abstract: 【OBJECTIVE】Effects on protective enzyme activities of tomato seedlings under different concentration of abscisic acid (ABA), putrescine (Put) and epibrassinolide (BR) were investigated. 【METHOD】The tomato seedlings were pre-treated with various concentration of ABA、Put、BR, and then treated with sub-optimal temperature stress in the growth chamber. 【RESULTS】The results showed that under sub-optimal temperature the content of MDA increased; SOD activity raised firstly and then decreased. Pre-treated with ABA, Put or BR can decrease MDA content and improved protective enzyme activities. The appropriate concentration which deceased MDA content most is ABA1(0.2mmol/L), Put2(0.5mmol/L) and BR2(0.01mg/L).The most suitable concentration to SOD activities is ABA1(0.2mmol/L), Put3(1.0mmol/L) and BR2(0.01mg/L). Compared with the control the activity of SOD increased in 29.2%、17.4%、28.6% respectively and MDA content decreased in 47.7%、43.2%、42.2% respectively. 【CONCLUSION】The tomato seedlings were hurt by sub-optimal temperature. ABA、Put、BR played a positive role in alleviating the effect of sub-optimal temperature on tomato seedlings.