In flood control calculation, the dependability of design flood directly affects the scale, safety and waste of the schedule engineering. The paper mainly elaborated the several wrong regions that frequently influence design flood calculation results. They are that the first is to use more data of interpolation and extrapolation flood series,the second is that the more the number of historical extraordinary flood events，the better,the third is the use of hydrological analogy method of area and length of river,the last is that without great continuous floods, the design flood doesn't change and so on.Taking the flow series of Zhangjiashan and Taoyuan hydrology stations of Jing River in Shaanxi Province for example, the paper analyzed the impact of the four wrong regions .