According to the survey conducted from Jan. 2004 to Dec. 2005, the seasonal characters of nitrogen in the water of input rivers of Three Gorges Reservoir (TGR) were discussed, and the forms and distributions of nitrogen among three major input rivers (Changjiang river, Jialingjiang river and Wujiang river) were studied. The results indicate that some primary hydrologic values of input rivers have seasonal characters, and they are in the danger range, which might bring algal bloom. The average contents per year of total nitrogen are 1.55-2.15 mg/L in the three monitor sections of input rivers of TGR, and Wulong monitor section is more severe than the others. The contents of total nitrogen in abundant water period are much higher than them in low water period, which displays the nonpoint source pollution has far-reaching influence upon total nitrogen. The total dissolved inorganic nitrogen (DIN) is the major form of total nitrogen and further nitrate nitrogen (NO3(-)-N) is dominant accounting for 70% or more of DIN. Ammoniacal nitrogen is the uppermost input form of nitrogen pollution of input rivers of TGR. Ammoniacal nitrogen was oxidated to nitroso-nitrogen, and then it was oxidated to nitrate nitrogen using up a lot of dissolved oxygen of water. Nitrate nitrogen sources include agricultural runoff, city pollution water, city runoff and slow-releasing from submersed soil, whenas nitrogen-ammonia (NH4(+)-N) origins are city pollution water, industrial pollution water, and a little source of living garbage and bilgewater.