Abstract: The seasonal and interannual variation of inflow runoff to Danjiangkou Reservoir, and their effects on the South-to-North Water Transfer Project were analyzed by using Mann–Kendall method, Markov process and runs theory. The results show that: (1) The annual inflow runoff volume displayed a decreasing trend. The average annual inflow runoff of 1999 -2017 decreased by 5.05 billion cubic meters compared with that of 1956-1998, which was mainly caused by the runoff reduction in flood seasons. (2) In terms of the annual inflow runoff transformations, the following types occurred more frequently: wet years to wet years, wet years to dry years, normal years to dry years and dry years to normal years, and the occurrence probabilities of those transformations are all more than 40%. (3) It rarely happens that wet years，normal years and dry years last 4 years or longer. However, it is still necessary to take continuous dry years into consideration in making reservoir operation strategies.