Field plot experiment with two nitrogen (N) applying rates was conducted to study the effect of organic addition on N leaching in rice season of Taihu Lake Region, and the mechanism was also studied through nitrate (NO₃^- -N) migration of water and soil. The designed treatments included no nitrogen (CK), conventional chemical N fertilizer (CT, N 300 kg/hm²), reducing N (RT, N 225 kg/hm²), CT+ rapeseed cake fertilizer (CT+M, M 2 250 kg/hm²), RT+rapeseed cake fertilizer (RT+M). The results showed that: 1) There was no significant difference in soil NO₃^- -N leaching between organic addition and inorganic fertilizer treatments at 30 cm depth during the two rice seasons of 2014 and 2015 except CT treatment in 2015. NO₃^- -N leaching at 80 cm depth in CT+M was 41% lower than that in CT, and RT+M produced 12% lower NO₃^- -N leaching loss compared to RT. 2) Floodwater NO₃^- -N under chemical N treatments was significant correlated with soil NO₃^- -N leaching, but the involvement of organic fertilizer weakened the correlation. 3) Organic addition increased soil organic matter content (SOM), and CT+M achieved 6.7% higher than CT. Organic N treatments showed higher soil NO₃^- -N content at 0–20 cm layer compared to chemical N treatments. And soil NO₃^- -N content at 20–40 cm layer differed little between organic N and chemical N treatments. These indicated that organic addition enhanced the absorption and fixation of NO₃^- -N by soil through the improved SOM and thus inhibited the migration of NO₃^- -N to deep soil. The reduced NO₃^- -N leaching under organic addition was not achieved by reducing floodwater NO₃^- -N concentration. Additionally, organic addition improved soil quality and rice yield, promoted crop N uptake, and thus could also explain the reduced NO₃^- -N leaching loss. The results provide scientific basis for decreasing NO₃^- -N leaching in farmland.