Abstract: The flood quantile estimated by conventional seasonal floodfrequency is less than or equal to annual maximum estimator,and can not satisfy a given flood prevention standard.A bivariate joint distribution with Pearson Type Ⅲ distribution margins is developed based on Gumbel-Hougaard Copula and used to describe two seasonal maximum flood series.A relational equation between annual maximum flood and seasonal maximum floods is developed,and the relationship between the frequencies of seasonal flood quantiles and design flood prevention standard is also given.Ultimately a new seasonal flood frequency analysis method is proposed and developed.Compared with the conventional annual maximum method,the seasonal design floods estimated with the proposed method are slightly greater in the main flood season and less in other flood seasons.It is shown that the proposed method could satisfy flood prevention standard and provide a new way for determining seasonal design floods.