To investigate the functional mechanism of Mg to cucumber under low temperature, we studied the effects of Mg stress on photosynthetic character and active oxygen scavenging system in cucumber seedlings under different temperatures using artificial climate box. The results showed that the value of Chla/Chlb was increased under Mg stress condition no matter what temperature, and increased more under Mg deficiency than under Mg excess. The contents of Chla, Chlb and carotinoid were decreased under Mg deficiency at both temperature levels, and under Mg excess at low temperature, but increased at suitable temperature. The Pn, Gs and Tr were decreased under Mg stress at both temperature levels with the lowest value observed under Mg excess and low temperature. The effects of Mg stress on Ci, WUE and Ls were different depending on temperature and Mg concentration. The main limiting factor of photosynthesis in cucumber seedling was non-stomata factor under Mg deficiency, while it was stomata factor under Mg excess. The O2- producing rate, H2O2 and MDA contents were increased under Mg stress in which the effect under Mg excess was higher than under Mg deficiency. Low temperature accelerated the Lipid peroxidation. Under low temperature, POD activity were increase, SOD activity decreased, but CAT activity were not significantly affected; under suitable temperature, Mg stress made POD activity increase, CAT activity decrease, but SOD activity did not change. At the suitable temperature, plant mainly alleviated Mg stress by increasing POD activities. At the low temperature, Mg deficiency had more severe damage on plant, which might be related to the increase of activity oxygen system.