Distributional changes of annual runoff in Beiniuchuan River basin were studied to provide a scientific basis for water resource planning, management and exploitation. [Methods] Based on the observed monthly streamflow in Xinmiao station from 1966 to 2010, annual distributional unevenness and concentration degree were calculated. Then Mann-Kendall test and cumulative filter method were employed to detect the trend of intra-annual runoff. [Results] (1) Generally, seasonal runoff distribution was bimodal with two peak values in August and March. The annual runoff distribution was very uneven with its maximum in August, which accounted for 37.63% of annual runoff. The minimum runoff in January only accounted for 0.65% of it. (2) The unevenness and concentration degree of runoffs in 1960s and 1970s reached the maximum, and they hit the lowest in 1980s. The maximum runoff occurred in late July to mid-August, relatively concentrated. (3) Monthly runoff had an obvious declining trend from 1966 to 2010. [Conclusion] Resulted from climatic change and human activity, the annual runoff showed an uneven distribution and the monthly runoff had a declining trend.