In the context of global warming, the ecosystem process and security in the source region of the rivers were focused in the paper. As nitrogen is a predominant element for ecosystem function and process in Qinghai-Tibet Plateau. the NPP of alpine grassland is strongly affected by the cycle of nitrogen. Thus analysis of the balance and budget of nitrogen in ecosystem process is very important for us to understand the vulnerability of plateau in the context of global change. Based on the the data of precipitation, runoff and sediment, we set up a simple budget cycle process of the source region of Changjiang River. In this cycle process, whole nitrogen cycle were divided into two parts, nitrogen input and output respectively. The landuse scene shows that there are no agricultural activities in this region, so man-made nitrogen source which from chemical fertilizer was not considered in the paper. Meanwhile we regarded the ecosystem process as a black-box, which means that processes such as leaching, denitrification etc were not considered in this study. The total amounts of nitrogen wet deposition can be estimated by the observed product of precipitation and nitrogen wet deposition. The ratio of wet deposition and dry deposition in China was cited to calculate the total amounts of nitrogen deposition. The nitrogen amount which is delivered by runoff was calculated through the product of runoff efficient and the total nitrogen amount. The total nitrogen amount in sediment was estimated by the product of sediment discharge and soil nitrogen content on the base of the same of nitrogen content between sediment and soil. The solved amount of sediment nitrogen in runoff was calculated by the difference between nitrogen amount delivered by runoff and nitrogen amount in the runoff, which can be got by calculating the product of total runoff and water quality data. Then differences of nitrogen input between inland and plateau were discussed. It shown that the lack of exogenous nitrogen input causes nitrogen input lower in plateau than that in inland, which cannot cause the nitrogen environment overload in plateau. The relative research shown that the denitrification is much higher than the nitrogen input, and the ratio of leguminous grass is low in the pasture It result in nitrogen fixation less than the nitrogen loss. In the paper, the following conclusions were pointed out: (1) The nitrogen deposition occurs mostly in summer, and the amounts account for 70% of whole year. The amount is almost the same between spring and fall. (2) The wet nitrogen deposition amount are 0.290-0.307 g N/(m2?a). The total nitrogen deposition are between 0.345 g N/(m2?a) and 0385 g N/(m2?a). Therefore The actual nitrogen deposition which deduct the nitrogen delivered by runoff are from 0.275 g N/(m2?a) to 0.315 g N/(m2?a). (3) Sediment is a key factor for nitrogen loss, which deliver 0.606 g N/(m2?a) on average. 10% nitrogen in sediment is dissolved in the runoff. (4) The nitrogen budget is -0.306 g N/(m2?a) which caused by precipitation, runoff and sediment. It shows that the source region of Changjiang River is a nitrogen source.