The time series analysis is an common method of data analysis in stochastic of hydrology. It uses frequency analysis to compute the periodic change rule of hydrological time series. The basic concepts and theories of frequency analysis have been introduced to get the period of the discharge of Danjiangkou Reservoir time series based on the hydrological observation records of Danjiangkou Reservoir from the year of 1930 to 2005, and tested it by X~2 distribution method. The results show that Danjiangkou Reservoir has about 22 years and 8 years period. It will be used to reveal the law of hydrological period in the upstream of Danjiangkou Reservoir, and provide reference for water resources scheduling of Middle Route of the Southto-North Water Transfer Project.