Abstract: The agro_system of a semi-arid belt is characterized by strong fluctuation. Based on the power spectra, precipitation and number of livestock only cycle in phase in three-year and six-year quasi-periods in the study area. The Cυ for the protein and calorie yield decreases east, north and west from the highest region of upland, which reflects the spatial fluctuation of the agro-system. The transfer of fluctuation of factors driven by rain is as follows: in the whole area the Cυ of yield for grain, calorie and protein is 1.5 times higher than 23％ of rain, that for livestock accounts for only 60% of the rain, but the Cυ for the output value of agriculture maintaining the same level as rain is lower than that of the yields above. There is a spatial difference in Cυ between the north and the south. The Cυ of the output value of agriculture in the pastoral zone is only half that of the rain, but in the agro-pastoral interleaving zone it is 1.6 times higher than that of rain. The correlation analysis also shows that precipitation and grain output influence the change in livestock during both this and next year.