As the water source of mid - route of the South - North Water Transfer Project, the changing tendency of the inflow of Danjiangkou Reservoir has very great effect on the construction, operation and management of the project. The Mann - Kendall method and linear - regression method are used to analyze the changing tendency of the precipitation over the upstream of Hanjiang River and the crude inflow of the Danjiangkou Reservoir. The calculated results show that there is a mutation from ample flow to low water in 1991 for both the precipitation and the crude inflow, but the long - term changing tendency of both the precipitation and the crude inflow is not distinct, namely, the phenomena of durative low water in 1990s just was a low water period of ample - dry water alternation in the Hanjiang River catchment. On the based of the predicted water consumption overall the upstream, the inflow of Danjiangkou Reservoir is about 356.3×108 m³ in 2030, it means that the Danjiangkou Reservoir still will have the capacity to provide water for north China.