The Weihe River is the greatest river in Shaanxi Province.Its water pollution becomes very serious in recent years and has restricted the development of local economy.The temporal and spatial characteristics of variation in the pollution indexes of COD_(Mn) and NH_3-N,and the reasons for the variation were analyzed based on the data of water quality observed from 1991 to 2002.Results show that the Weihe River has been polluted seriously,in which the most serious COD_(Mn) pollution was observed at the Xianyang Railway Bridge,and the serious NH_3-N pollution was also observed at the Wolongsi Bridge,the Guozhen Bridge and Xingping City.Pollution of the river has become more and more serious since 1990's,with the highest value of NH_3-N observed in 1996 and the highest value of COD_(Mn),in 1995.The polluted water from paper mills is the main source of COD_(Mn) pollution,and chemicals applied on farmland and the discharge of waste water by fertilizer industries is the main reason for NH_3-N pollution.