Abstract: In order to determine physiobiochemical mechanisms of the first cropping rice and its ratoon rice under different N application modes in the first cropping rice, the different ratios of N supply for basal-tillering fertilizer to that for spike fertilizer in the first cropping rice were designed under total N supply of 225 kg?hm?^-2, which were N1 (8︰2), N2 (7︰3), N3 (6︰4), and zero N supply (N0) was the control. Changes in nitrate reductase activity, chlorophyll content, net photosynthetic rate, and root activity were analyzed and their effect on dry-matter accumulation and grain yield formation determined. The results show that the changing tendency in nitrate reductase activity, chlorophyll content and net photosynthetic rate in different treatments is similar under different N application modes in different developmental stages of the first cropping rice and its ratoon rice, showing increasing at the early stage, peaking at the booting stage, then gradually declining in the first cropping rice. It rapidly ascends again at full- heading stage, and then rapidly declines at the ripening stage of the ratoon rice. It is also found that bleeding rate continuously declines after the booting stage of the first cropping rice, however, during the ratooning cropping, it gradually increases at budding and tillering stages, reaches a peak at full-heading stage, then gradually declines at maturity. The result indicates that nitrogen supply enhances nitrate reductase activity, chlorophyll content, net photosynthetic rate and root activity. This therefore suggests that the proportion of N supply should be in-creased to enhance these parameters during the middle and late growth stages of the first cropping rice. The good pratoce in N application can also enhance root activity, especially at the booting stage of the first cropping rice. This, in turn, can result in in-creased dry-matter accumulation of the first cropping rice, consequently, high grain output of the ratoon rice .