The Changjiang estuary is a typical estuary that fluvial factors plays a dominant role in fluvio-marine interaction. The process and its variation of runoff and sediment discharge have situation under control in respect of evolution of the estuarine environment. This paper studies main characters of runoff process at Datong Station with methods of time-series analysis. It is stated that the runoff series at Datong Station has little change trend since 1923, but a jumped decrease near 1955 and a jumped increase near 1988 have emerged. Meanwhile, it has a certain appearance of periodic variation with around 16,7 years. The distribution patterns of monthly flow, the matching state of high or low water between flood and dry season as well as the relevant causes are also discussed in the paper.