Abstract: By means of frequency spectral analysis method,the authors use the Gamma ray curves of fluvial depositional system growing carbonaceous marsh to analyze the sedimentary cycle of Y2 member,Yan'an Formation in Longdong area,Ordos Basin,reaching that sedimentary cycles having stable thickness are conserved very well in the strata.The ratio of the thickness of the sedimentary cycles corresponds with the ratio of Milankovitch cycles.Therefore,it is reasonable to deduce that the sedimentary cycles are controlled by palaeoclimate,which caused by the Milankovitch cycles,and its cyclical transformation determined the formation and the evolution of high frequency sedimentary cycle in marshland or flood plain deposit and transitional facies of river lake alternately appearing.The thickness ranges of sedimentary cycles caused by the eccentricity cycle,the obliquity cycle and the precession cycle are respective 8.72~11.29 m,2.54~4.3 m and 1.23~1.91 m.The thickness of the stratigraphic cycles including coal and carbonaceous shale is about 10 m,and it is controlled by the eccentricity cycle,which is 123 ka.Gamma ray curves including a great amount of geological information reflect well stratigraphic cycles caused by climate changes.