Abstract: In order to improve the purification efficiency of different forms of nitrogen in farmland runoff by constructed wetlands (CWs) around the paddy field, and effectively intercept nitrogen (N) and phosphorus (P) runoff loss caused by non-point source pollution in farmland, agricultural waste rice straw was used as organic carbon source to enhance the purification capability of CWs planted with submerged plant Vallisneria natans (Lour.) Hara. Four kinds of CWs were set up: without submerged plant (NS), only planted with V. natans (VN), only added with rice straw (SS) and with V. natans planted and rice straw added (VS). The purification performance and mechanism of CWs were studied when treating farmland tail water with ammonia nitrogen (NH₄ ⁺-N) or nitrate nitrogen (NO₃ ⁻-N) as the main N components. The results showed that the purification efficiencies of TN and NH₄ ⁺-N in VN and VS were significantly higher than those of NS and SS (P<0.05) in the treatment of farmland tail water with NH₄ ⁺-N as the main nitrogen form, which indicated that planting V. natans played a crucial role in the removal of NH₄ ⁺-N. The average removal rates of TN and NO₃ ⁻-N by SS and VS were significantly higher than those of NS and VN CWs (P<0.05) in the treatment of farmland tail water with NO₃ ⁻-N as the main nitrogen form, suggesting that the addition of rice straw significantly improved the reduction of NO₃ ⁻-N. CWs only planted with V. natans had poor removal efficiency on NO₃ ⁻-N. While the removal of NH₄ ⁺-N was limited in CWs only added with rice straw. VS showed excellent purification performances on the removal of two forms of N. At the same time, VS showed similar TP removal efficiency to VN. The above results suggested that it was a feasible way to apply this kind of CWs to control agricultural non-point source pollution.