Abstract: In this paper a diagnostic analysis has been performed by using the data of monthly mean runoff of Tangnaihai hydrometric station in the upper reaches of Yellow River from 1956 to 1994.It is shown that there is a good persistence in winter half year especially,its significant lag correlation can reach more than 6～8 mont hes from Automn to nextearly Summer,the inflexion period is from Aug.to Sept.In recent 40 years,the runoff evolution was a poor to rich to poor processes.The anuual mean runoff appeared obviously 7 year period and 14 year oscillation in Automn.The probability of the flow-off in each rank is forty-one percent of low flow years,the anomaly year can reached two-third.Precipition over Tibetan plateau in summer and Automn have obviously contribution to runoff at the contemporary and lag half year.The early thermal for cing over tibetan Plateau and feature of general circulation have an indication to the runoff forecast in Summer and Automn.