Precipitation fluctuation is one of the main indicators of climate change,which plays a significant role in driving various vegetation processes in the semi-arid desert steppe.Due to change trends of annual precipitation and growth period precipitation,we analyzed the responses of different vegetation coverage area changing process to precipitation fluctuations by building ground reflectance spectrum model and predicting regional vegetation coverage distribution pattern based on"3S" technology.The results showed that vegetation coverage areas were fluctuated with the annual and growth period precipitation.The relations between vegetation coverage areas and annual and growth period precipitation were fitted through four kinds of functions which were linear,power,exponential and logarithm function.Vegetation coverage area around 10% could be considered to be the thresholds of whether vegetation coverage distribution areas were increase or decrease in semi-arid desert steppe.And it would be decrease when vegetation coverage was less than10% with rainfall increase and other was versa.The functions simulated the trends between different vegetation coverage areas and precipitation fluctuations were better fitted when the vegetation coverage area were at the level of<2%, 2%-6%, 10%-15%, 15%-20%and20%-25% (p<0.01).The fluctuation was related to the massive growth of different desert perennial grasses,such as Allium polyrhizum,Heteropappus altaicus,Salsola laricifolia,and Eragrostis minor Host during yearly(or seasonal)ample precipitation.