The effects of different nitrogen levels on the carbon-nitrogen metabolism and yield of relaycropping soybean under maize/soybean pattern were studied. The results showed that increasing the supply of nitrogen properly was favorable to get high yield of soybean, but excessive nitrogen caused serious yield loss.Under high nitrogen levels（supply pure N 78.2 and 96.6 kg per hectare）, the accumulation of soluble sugar and starch in leaf and stem were reduced, and accumulation period of starch in stem was shortened; excessive nitrogen accumulation in stem and leaf lead to excessive nitrogen metabolism, which aggravated photo-inhibition and contribute tometabolic disorders, so the activity of SPS and GS kept at a low level, the transfer and distribution of nutrients for seeds were blocked. In conclusion, the nitrogen supply of relay-cropping soybean should be based on investigation of soil N content. In this field experiment, soil organic matter content was 53.106 g·kg^-1,soil total N content was 2.15 g·kg^-1, soil available N content was 158.542mg·kg^-1, the supply of 0~59.8 kg of pure N per hectare was beneficial to soybean yield formation, but there was no significant difference among 4 treatments.In production of high soil fertility,Nsupply was need not for saving costs.