The effects of chlorpyrifos and exogenous salicylic acid on anti-oxidant defensive response of Brassica chinensis were investigated by indoor cultivation experiments. The activities of superoxide dismutase (SOD) and catalase (CAT) in B.chinensis were suppressed significantly, whereas the activity of peroxidase (POD) was increased significantly within 4 days after being sprayed with chlorpyrifos. In addition, the contents of malondialdehyde (MDA), proline and glutathione (GSH) significantly increased in a short period (2-4 day) after the spraying. These results indicated that the usage of chlorpyrifos caused stress on B.chinensis seedlings. The inhibitions of chlorpyrifos on activities of SOD, CAT and GSH could be relieved by high concentrations of exogenous salicylic acid, whereas exogenous salicylic acid almost had no influence on the activity of POD and the contents of MDA and proline. These results suggested that exogenous salicylic acid only relieved stress on B.chinensis to some extent.