The study area is located in the center of Dalate County. Based on 3S techniques and combined with ground investigation, the Land-sat TM images of 1987, 1995, 2000 and 2003 were interpreted. The changes of sandy land were analy zed by using the database of desertification dynamics from 1987 to 2003. RS and GIS softwares, such as Erdas 8.6, ArcView 3.3 and ArcInfo 7.1 were used as an interpretation tool. The cumulative impact analysis was used to find the reasons of desertification. Results showed that in the past 20 years, the desert-if ied lands were in the status of the reverse-development-reverse. The degree of dynamic desertification was the highest from 1995 to 2000. The analyses of natural elements and human activities with sliding average indicated that the cumulat impacts of population and livestock on desertification were signif icant, and the effect of population was strong er than livestock. the cumulative impacts of annual mean wind were always significant in all temporal scales. The effect of annual mean temperature was significant (at 0.01 scale) over two years.