Identifying the characteristics of nitrogen output during the paddy field drainage period was critical to control of nitrogen pollution output in the reclamation paddy area. In this study, a paddy field and ditch system in Abujiao basin was selected as the study area. Combined with isotope tracer technique, the characteristics of nitrogen output and the reduction mechanism of nitrogen in paddy fields and ditch systems during the paddy drainage periods were systematically studied. The results showed that ponding drainage period was the most significant period for nitrogen output from paddy fields, mainly controlled by chemical fertilizer application. During this period, total dissolved nitrogen, ammonia nitrogen and nitrate nitrogen discharge load accounted for 42.57%, 69.73% and 54.89% of the year's total amount, respectively. There are distinct differences of nitrogen reduction mechanism in paddy fields and ditch systems for three drainage periods, indicated by results from variation characteristics of nitrogen and nitrate isotopic tracers. During the ponding drainage period, the ammonium nitrogen decreased significantly, almost half ammonium nitrogen reduced during six-day sampling period. Isotopic tracers showed that the removal process was mainly controlled by the nitration process. During the rainfall drainage period, the nitrate nitrogen concentration decreased by 24.00%, while the ammonium nitrogen increased by 26.04%. Mixed with rainfall (high δ ¹⁸O-NO₃^- values and low nitrate concentration) might explain the elevated δ ¹⁸O-NO₃^- values and linear reduction process of nitrate in the ditch. The fluctuation of ammonium nitrogen might be linked to the input of ammonium nitrogen in paddy fields and the minerali-zation of soil nitrogen. During the sunning drainage period, the reduction processes of nitrogen were more complicated, and isotopic tracer signals were not obvious. The ammonium nitrogen concentration decreased by 52.07%, which might be associated with absorption by vegetation.