Based on the historical data of drought and flood from 1470 to 1979,the grade data of drought and flood are obtained and then extended to 2003. With the methods of wavelet analysis, t-test and low-pass filter,the occurrence laws,periodicity, phases and sudden-change of drought /flood are analyzed for the last 543years in Zhejiang. The results show that (1) Middle-intensity drought/flood events occurred every 3—4 years and serious drought/flood occurred about every 10 years. (2)Zhejiang drought/flood has obvious 30-year,50-year and 70—80-year oscillation periods,then about 10—20-year. (3) The fluctuation of drought/flood appears three ascending stages mainly characterized by much drought and three descending stages mainly characterized by much flood. (4)Drought/flood have three obvious sudden-change points including 1636,1708 and 1963, which represent climate evolution characteristics from lack of rain, stage of rich rain to stage of lack of rain.