There is a dispute about existence of a long period from Eastern Han Dynasty to Tang Dynasty (90-870 A D) in history,when flood was rarely on the lower reaches of Yellow River.It was considered that the long period without severe flooding on the lower reaches of Yellow River during 70 -870 A D is assuredness based on the historical recordation and by using the disaster system analysis.The cause of rare flooding on the lower reaches of Yellow River was resulted from the integrated effect of multi-factors.The main causes include three factors as following.(1) The water quantity reduction of lower reaches of Yellow River results in the cold - dry climatic period,and the decrease of the loss of water and soil owing to the change of land use mode in the Loess Plateau which lead lessen of sandy quantity in the lower reaches of Yellow River.(2) Greatly improve the prevent flood ability because of the control engineerings of Yellow River and there are lots of lake and swampy in the lower reaches of Yellow River.(3) The sparse population of North China Plain was resulted from war, and conduce the lower on the social frangibility.