The theoretical frequency distributions of P-Ⅲ, Log P-Ⅲ and GL (Generalized Logistic Distribution) were compared and analyzed by using annual maximum sampling and L-moment method in the river basins of the Huaihe and Pearl, where there are 18 and 16 hydrometry stations with data series (1951-2010) collected, respectively. The results show that the design flood from the curve of P-Ⅲ is less than that from the other two curves and the errors of P-Ⅲ are less than these of log P-Ⅲ by analyzing the errors of the curve fitting in these two basins.