Drought and flood are the main natural disasters surrounding Lanzhou area. We described the variation characteristics of drought & flood grades of Lanzhou area from 1470 to 2008 by performing the manners of accumulative departure analysis, running T-test and Morlet wavelet analysis. Results showed that the drought & flood severity has 3 obvious drought stages and 4 flood stages nearly the 539 years. The frequency of drought is larger than flood. There are two abrupt changeson 100-year scale and 14 abrupt changes on decadal scale in Lanzhou area. Positive and negative phrase appear alternately during different time scales. The major cycles of drought & flood grades transformation are 108 years, 39 years and 13 years, respectively. With the rising temperatures and decreasing precipitation, the drought & flood had been showing the dry trend from 1951 to 2008.