Tennant method was widely applied to river ecological runoff calculation, but it was very difficult to confirm perennial average flow by this method, and the average sometimes did not accurately reflect the central tendency of overall distribution. To deal with this problem, the statistical character of the practical data was studied and the median and mode theory was introduced to avoid extreme data effect to statistics by improved Tennant method, and the typical flow process was finally found out. The application on ecological runoff calculation of Luan River downstream and the comparison between this method and old ones showed that the improved method was much more reasonable.
