In order to study flood-damage rules,it is necessary to recognize the history of flood. According to the historical records, the occurrence rules of dry and flood for both Yangtze River and Yellow River were analyzed. Based on the dry and humid changes of climates in recent 1000 years in the Yangtze Basin, the periodic dry-flood variations were discussed and the flood frequencies and disasters from 1840 to present were analyzed, flood characteristics and relieving measure were proposed. The study shows that the probability of simultaneow occurence of flood in the two basins are low but the probability of dry are high; the flood frequency is higher than dry frequency in Yangtze River basin; the flood frequency in Yangtze River is higher than in Yellow River; flood frequency in middle and lower reaches of Yangtze River is higher than that in its upper reach.