As themain engineering means of controlling urban flood.river and upstream reservoir regulations which protect people against flood disasters can change river channel storage and lead to the variations of the formation, evolution, and spatial and temporal distribution of urban flood due to climate change. In this study, hydrological variation diagnosis system and the formula of Chezy-manning were used to determine the flood variation form,degree,and causes of Xuanen district under the changing environment. The results showed that (1) the average annual maximum monthly rainfall series from 1956 to 2010 have a moderate variation with downward jump in 1998, which indicates that rainfall decreases under the changing environment; (2) the annual maximum daily flow series from 1962 to 2013 have a moderate variation with downward jump in 1999,which indicates that peak flow decreases; (3) The dual effects of climate change and human activities cause the flood variation,but the former effects are only 41% of the latter; and (4) river canalization results in the decreasing of the flood flow and exacerbation of flood variation, therefore the flood warning plan needs to adapt to the changing environment.