Through pot cultivation experiments of Trifolium repens L under Cu stress,we studied the effects of Alternaria tenuis Nees on the physiological and biochemical characteristics of two different populations of Trifolium repens L selected from a Cu-polluted area (marked as populationⅠ) and a non-polluted area (marked as population Ⅱ).The results showed that with Cu treatment, the infection of A. tenuis increased leaves’ electric conductivity and MDA content of two populations with Cu concentrations from 0～1500mg•kg^－１,while electric conductivity was lower and MDA content higher than that of population Ⅱwhen Cu concentration was 2000mg•kg^－１.The infection also decreased chlorophyll and protein contents. In two inoculated populations, chlorophyll and protein contents declined with the increasing concentration,and chlorophyll content was negatively related to Cu concentration (correlation coefficients of populationⅠandⅡwere －0.954* and －0.961**).When Cu concentration was low (0 and 500 mg•kg^－１),the SOD activity in two populations of inoculated plants was higher than that of non-inoculated plants. But when Cu concentration was high the inoculated plants’ SOD activity was lower than that of non-inoculated plants. Similarly, under low levels of Cu (<500mg•kg^－１) POD activity was higher of inoculated plants than that of non-inoculated plants. After the increase of Cu concentration POD activity declined rapidly and was lower than that of non-inoculated plants. Finally, CAT in two inoculated populations was higher than in non-inoculated ones from 0～1000mg•kg^－１,but lower from 1500mg•kg^－１ in populationⅠ,whereas in populationⅡit was lower at 2000mg•kg^－１.The sensibilities of the protective enzyme also differed from each other according to the Cu stress.The order was POD>SOD>CAT. The results also indicated that under the stress of a high Cu level(2000mg•kg^－１), the decline of antioxidative enzyme acitivities broke the normal balance between production and elimination of acitive oxygen, resulting in the accumulation of excessive active oxygen which aggravated lipid peroxidation, therebycausing toxic effects on T. repens.And within the inoculation with A. tenuis,these toxic effects were more serious. Population Ⅰcould subsist within 2000mg•kg^－１ Cu in soil, but when infected with A. tenuis the concentration should be less than 1500 mg•kg^－１.In comparison, population Ⅱ could subsist within 1500mg•kg^－１ Cu in soil but within 1000 mg•kg^－１ under the stress of Cu together with A. tenuis.However,compared with population Ⅱ from the non-polluted area,the T. repens in population Ⅰ from the Cu-polluted area showed more resistance to the stress of Cu and A. tenuis.