In order to investigate the physiological responses of castor bean under heavy metal Cd stress，a greenhouse pot experiment was conducted to investigate the effects of the different concentrations of Cd2+(0，20，50，90，100，200 and 300 mg/kg)on the photosynthesis，the activity of protective enzymes(SOD，POD and CAT)and the ascorbic acid-glutathione(AsA-GSH)cycle in castor bean seedlings under stress.The results showed that the change of chlorophyll content had no obvious regularity with the increase of Cd concentration，but when the Cd concentration was more than 100 mg/kg，its value decreased significantly；As the Cd concentration increased，the photosynthetic rate(Pn)，stomatal conductance(Gs)，transpiration rate(Tr)and intercellular CO₂ concentration(Ci)all showed a trend of first increasing and then decreasing.When the Cd concentration reached 200 mg/kg，except for Tr and Ci，all other indicators were decreased significantly compared to the control(P<0.05).Cd stress led to the increase in the content of Malondialdehyde(MDA)in the leaves of castor，and the content of MDA in the roots increased first and then decreased.The activities of superoxide dismutase(SOD)，peroxidase(POD)and catalase(CAT)in leaves and roots increased first and then decreased.When the Cd concentration reached 200 mg/kg，except that the POD of leaves was higher than the control，the other indicators were lower than the control.In the AsA-GSH cycle，with the increase of Cd concentration，the activities of ascorbate peroxidase(APX)，glutathione reductase(GR)and the contents of AsA，GSH increased first and then decreased.When the Cd concentration reached 200 mg/kg，except the AsA content in leaves and roots and the APX activity in roots were lower than the control，the other indexes were higher than the control.Results indicated that Cd stress induced a significant increase in the antioxidant system activity，improving their antioxidant capacity of castor.However，as the concentration increased，the activity of antioxidant enzymes and the content of antioxidant substances decreased，causing toxicity to the plants.Moreover，plants mainly depended on AsA-GSH cycle to resist the toxicity of ROS under severe stress.