The response of leaf antioxidant enzyme activity，yield and oil content to nitrogen fertilizer level during grain filling was investigated by using field experiment with randomized block design，taking dwarf castor hybrid Tongbi 11 as test material and high-stalk hybrid Tongbi 9 as control under 4 nitrogen fertilizer treatments of N 0，60，120，180 kg/hm². The results showed that the activities of superoxide dismutase (SOD)，peroxidase (POD) and catalase (CAT) increased continuously and the content of malondialdehyde (MDA) decreased continuously with the increase of nitrogen application level in the range of 0～120 kg/hm². When nitrogen application level increased to 180 kg/hm²，SOD，POD，CAT activities decreased，while MDA content slightly increased. The activities of SOD，POD and CAT of Tongbi 11 were higher than those of Tongbi 9 at various nitrogen fertilizer levels and filling stage，and the MDA content was lower than that of Tongbi 9. Under low nitrogen level treatment，most of the differences between the two varieties were not significant，while under high nitrogen level treatment，most of the differences were significant. With the grouting progress，SOD and POD activities showed a single peak curve，but the peak value of Tongbi 11 was delayed compared with Tongbi 9. CAT activity decreased and MDA content increased. Grain yield，oil content and antioxidant enzyme activity of each treatment showed the same trend. Correlation analysis results showed that grain yield and oil content were positively correlated with SOD，POD and CAT activities，and negatively correlated with MDA content. In the early stage of filling，the correlation between SOD，POD and CAT activities of Tongbi 9 and grain weight reached significant or extremely significant level at 10 d after anthesis，while Tongbi 11 reached a significant level. The correlation between SOD，POD and CAT activities of Tongbi 9 and grain weight did not reach a significant level at 20 d after anthesis，but those of Tongbi 11 reached a significant level. In the middle stage of filling，the SOD and POD activities of Tongbi 11 were significantly positively correlated with oil content，while the oil content of Tongbi 9 was only significantly positively correlated with POD activity，and the correlation with SOD and CAT was not significant. The results showed that proper application of nitrogen fertilizer could improve antioxidant enzyme activity and reduce MDA content in castor main stem leaves. The protective enzyme activity of Tongbi 11 has a more positive physiological response to nitrogen fertilizer and has a stronger ability to improve the balance between production and removal of active oxygen in cells，which is the physiological reason why its grain yield and oil content are higher than those of Tongbi 9. The application rate of 120 kg/hm² nitrogen fertilizer is the best fertilization level for Tongbi 11 to obtain high quality and high yield in Xiliaohe plain.