Based on the runoff date measured by Maduwang hydrological station in Bahe river basin from 1956 to 2005, the paper synthetically used distribution non-uniform coefficient Cu、the distribution adjustment coefficient and the index of concentration degree CR to analyze inter-annual runoff change characteristics of Bahe River Basin on the basis,it used Mann-Kendall (MK) method to study trend and mutation of its interannual variability. The results show that: ① the annual distribution of runoff in Bahe River was relatively concentrated from 1950s to 1990s,and that in the 21st century is relatively uniform.The distribution non-uniform coefficient Cu was in accordance with the distribution adjustment coefficient;②Trend of Bahe River of annual runoff is significantly decreasing and the confidence level reached 99.7%;The decrease trend in winter is not significant in the other seasons is very obvious;③ The annual runoff mutations were in 1961 and 2002; There were three mutation in 1972、1977 and1987 in wet season and two mutations in 1977 and 1983 in dry season. and the trends of wet and dry season were consistent with the whole year.