By studying the feature of the curve of the Hurst exponent changing with the time, a method of calculating and judging the average "memorial" range of the chaotic time series, the mean period, is presented in this paper. When the curve is drawn according to the 7 steps give in the paper, you may find that there will be a peak value on the curve. The time point corresponding to this peak should be the mean period of the time series. Some results of the aircraft intactness ratio data by using this method have been approved by the related experts and can surely be used for the ulterior evaluation of the data.