Based on the wet and dry season nitrogen concentration data of the 4 south inflow rivers of the Xingyun Lake, thispaper studied the spatial and temporal changes in nitrogen concentration of different fractions, discussed the pollution reductioneffect of south rainwater regulation system, with a goal of providing scientific support for further control over pollutant input. Theresults show: (1) The change in total nitrogen (TN) concentration of the south rivers into the Xingyun Lake ranges from 1.1 to18.69 mg/L, and the TN concentration in rainy season is higher than that in dry season because of the farmland runoff pollution.(2) The N concentration of the south rivers into the Xingyun Lake is given priority to NO3-N, av eraging 39.49% of TN; theproportion of PN and NH3-N are similar, averaging 28.50% and 24.75% respectively; the proportion of DON is relatively low,averaging 9.97% of TN. The input amount contributions of NH3-N,NO3-N and PN increase significantly in the rainy season. (3)After regulating south rainwater system, the TN concentration of the 4 south inflow rivers show a trend of decline, reducing 32.35%on average. NO3–N reduction effect is most obvious, reducing 58.22% on average, in contrast to a relatively low reduction rate ofNH3-N and PN. (4) The rainwater regulate system should consider seasonal differences of the nitrogen pollution with various forms,and further combined with lakeside wetland to improve the reduction effects of NH3-N and PN.