Study on the characteristics and variation of drought and flood is very important for studying water resource evolution in a river basin. Historical documents and the records of two cities (Zhangye and Jiuquan) about watershed flood and drought were used and integrated to rebuild a drought and flood gradation sequence of the Heihe River basin from 0 to 1949 A. D.,through the methods of moving average and wavelet transform processing, to understand the frequency and cycle of drought in the past and their relationships with climate and human activities in the basin. The results showed that there were five drought phases (1230 - 1270,1430 - 1530,1640 - 1760,1860 - 1890,and 1900 - 1940) and three flood phases (1650 - 1690,1730 - 1790 and 1830 - 1910). From 1000 to 1949 A. D.,there were four quasi-periodic variations of drought and flood in the basin,which were closely related with solar activity. Before 1580,climate change was the main factor of drought and flood disaster. However,after the 16th century,the frequent events of alternate drought and flood seemed to be caused by human activities. Therefore,quantitatively analyzing the effects of natural factors and human activities on drought and flood disasters in the basin should be the focus of future research.