Abstract: Effects of different weak light levels on tomato physiological characteristics in blossom and fruit-setting period were studied by covering shading net. The results showed that when treated by weak light, the leaf area, chlorophyll content and net photosynthetic rate were decreased, causing yield reduction; the soluble sugar content in leaves was declined, MDA content increased, and soluble protein had no significant difference; and the activities of CAT, POD, SOD were all decreased. Different responses to weak light were observed in different tomato varieties. When treated by mild weak light (light intensity decreasing by 31.46%), yield of ‘Chaoyue’ reduced by 14.23%, showing high tolerance to weak light, and the photosynthetic ability decreased slightly, MDA content increased little, and the enzymes activities decreased little.