The identifying and ascertaining of cryptic period,includes simple period,complex period and approximate period,of hydrologic time series is a rather important and hard issue.A relatively perfect and effective means is made spectral analysis.MEM1(Maximum Entropy Method1)spectral analysis found on the principle of maximum entropy(POME)which overcomes many short of the traditional spectral analysis methods,has advantage such as slick spectrum and high distinguish rate.Therefore,MEM1spectral analysis was employed to detect the cryptic period of the annual runoff series,monthly runoff series,and maximum flood peak series of Huayuankou in Yellow River as the case.