Abstract: Based on the runoff data of Nenjiang River in 1956-2006,the Mann-Kendall,the wavelet wave analysis and the cumulative filter methods are used to find out the variation law and its impacting factor of runoff amount in Nenjiang River.The result shows that the runoff annual distribution in Nenjiang River is not uniform and occurs mainly in July,August and September.The result also shows that the runoff amount decreases in the recent 50 years,and more significant in the early of 1960s, while in the 1980s there is a trend of increase,but it isn't significant.In the middle and late of 1990s the runoff amount decreases again.The runoff changes from year to year with a change periodicity of 8 years.And other change periodicities are of 20 years and 40 years or longer.The most important climate factor is precipitation and evaporation.The human activity is also an important factor.