The traditional hydrological frequency analysis (THFA) method has been widely used to estimate the design flood for a given return period. It relies on the assumption that the series should be stationary. However, climate change and human activities caused the series to lose their stationarity,and thus limit the application of the THFA method. In this paper, we studied the530-year summer-precipitation series covering the period from 1470 to 1999. At the 0.01 significant level,it was found that the series had three change points: 1534,1724,and 1923. Therefore,the whole series was divided into four sub-sample series. We estimated the probability distribution function (PDF) of each sub-sample using the THFA method. Then we obtained the mixed probability distribution function (MPDF) by weighting and synthesizing the four PDFs related to the four sub-samples. The MPDF was regarded as the PDF of the whole series and was used to estimate the design precipitation for a given return period. Besides, the 530-year large precipitation series was also used to estimate the empirical value of the design precipitation. Then we compared the observation-sample-based estimations and MPDF-based estimations of the design precipitation. The results showed that the MPDF fit the series well.