In accordance with the analysis on the tidal level records more than100years in Huangpu River,there exist an obvious quasi-period cyclic.The Periodical Maximum Method therefore is approached.At the first,selecting basic cycling step length on the basis of periodic analysis,then,sampling with the period unit to carry out frequency analysis according to the random property of periodic maximum values,and by combining with the investigated historical extreme values.Since the extreme variables are all near or belongs to the heavy storm surge conditions,the extended rare occurrence frequency result,which is reasonable and reliable,can be used as the study reference for the design of large or middle hydro projects.