The effects of historical extraordinary flood events added to observed flood series on design flood, in four different cases mentioned in this paper, have been studied with statistical method. The results show as the following: (1)The longer return period of the events, the more stable and accurate is the design flood. (2)In the same return period, the effects of improving the precision or stability of design flood are quite visible when the observed flood series is added one or two extraordinary flood events, but invisible on the contrary when more the events are added. (3) Errors of the return period can not influence the events' improving the precision and stability of design flood. (4) Effects of value errors of the events on precision of design flood are more visible than their return period errors.