Abstract: The seedlings of ‘Xizhoumi No.25’ were used as experimental material, 0,5,10,20 mmol·L^-1 exogenous trehalose were pretreated for 3 days.The photosynthetic pigment content, net photosynthetic rate（Pn）,transpiration rate（Tr）,stomatal conductance（Gs） and intercellular CO₂ concentration（Ci） of seedling leaves were measured under low temperature stress of 15 ℃/10 ℃,10 ℃/5 ℃,5 ℃/1 ℃ for 1 day and 3 days.The effects of exogenous trehalose on photosynthesis of hami melon（Cucumis melon var.saccharins） seedlings under low temperature conditions were studied, and the mechanism of cold tolerance regulation was clarified, in order to provide reference for alleviating the damage of hami melon seedlings caused by late spring cold climate.The results showed that chilling stress markedly inhibited the photosynthesis ability of leaves compared to those of non-trehalose sprayed plants, moreover, the content of chlorophyll b（Chl b） increase significantly, the content of other photosynthetic pigments and Pn, Gs, Ci and Tr were decreased significantly as the temperature decreases; however, exogenously applied trehalose markedly alleviated the chilling-induced drastic effects on gas exchange attributes of hami melon seedlings under chilling stress.Appropriate concentration of trehalose could significantly increase the content of photosynthetic pigment and enhance the net photosynthetic rate, stomatal conductance and transpiration rate of melon leaves compared to non-trehalose-treated seedlings.Of the trehalose concentrations, the comprehensive evaluation of membership function method results indicate that the highest chilling tolerance was obtained with 5 mmol·L^-1 and 10 mmol·L^-1 trehalose pre-treatment.These results suggested that increased chilling tolerance might be occurred through trehalose significantly promoted the accumulation of chlorophyll a（Chl a）,Chl b and carotenoids, and improved the photosynthetic capacity under chilling stress.However, the high concentration of exogenous trehalose will reduce the mitigation effect of chilling stress.