Abstract: The nitrogen concentration in Miyun Reservoir is increasing, threatening the water quality guarantee for Beijing．Here we investigate the nitrogen concentrations and sources of origin in Banchengzi Reservior watershed, upstream of Miyun Reservoir．Nitrogen concentrations in flooding period were found to have increased markedly．Nitrogen load in the largest flood accounted for more than 90% of the whole year．During flooding periods, nitrogen concentrations in Mangniu River more than doubled that in Shizhuangzi River．Mangniu River was apparently the key catchment to control nitrogen output．Nitrate was found to be the major state of nitrogen, at an average percent of 74.3%, higher in rainfall seasons．Non-point sources were found to be the major source of nitrogen．At Banchengzi Reservoir watershed, natural factors including forests and rainfall were the major sources of nitrogen, the total percentage being >60%．The effect of human activities was less prominent, but mining was an important source of nitrogen increase．This work identified the major sources of nitrogen, and will be helpful for watershed management at Miyun Reservoir．