Abstract: Desertification is one of the most serious environment problems in the world and it does great harm to people's production and living conditions. With the implementation of Reform and Opening policy，Chinese economy has been developed rapidly and the desertification hazard in China has become increasingly serious. Mu Us Sandy Land locates on the farming-pastoral ecotone of Northern China，whose ecological environment shows significant sensitivity and vulnerability. It is very necessary for the managers to master desertification process in Mu Us Sandy Land over the past 40 years and carry out control programs. In this paper，Landsat images from 1977 to 2010 were used as remote sensing data and method coupled with decision tree classification and ISODATA unsupervised classification were used to retrieve desertification process in Mu Us Sandy Land over the past 40 years. Besides，annual average temperature and annual total precipitation from 1971 to 2010 were used to analyze the causes of desertification process. Results showed that：（1） Method coupled with decision tree classification and ISODATA unsupervised classification could be used for remote sensing interpretation in desertification area successfully. The accuracy of classification results was greater than 87.28%. （2） In Mu Us Sandy Land，drifting sandy land area in the late 1990s was the largest and the ones in the late 1980s and 2010 were the second largest and the smallest，respectively. Semi-fixed sandy land area in 2010 was the largest and the one in the late 1990s was the smallest. Fixed sandy land in 2010 was the largest and the one in the late 1970s was the smallest. （3） Over the past 40 years，desertification was the most serious in the late 1970s. The sum of areas of drifting and semi-fixed sandy land was 32 084.0 km2，which accounted for 83.64% of the whole region. The sums of drifting and semi-fixed sandy land areas in the late 1980s，the late 1990s and 2010 were 28 581.6 km2，29 576.3 km2 and 26 047.3 km2，which accounted for 74.51%，77.10% and 67.90% of the total，respectively. （4） Inter-conversions among drifting，semi-fixed，fixed sandy lands，farmland and water body were very active. New drifting sandy land mainly originated from semi-fixed and fixed sandy lands. From the late 1970s to the late 1980s，4 207.2 km² semi-fixed sandy land and 333.9 km2 fixed sandy land transformed into drifting sandy land. From the late1980s to the late 1990s，4 252.6 km² semi-fixed sandy land and 1 690.4 km2 fixed sandy land transformed into drifting sandy land. From the late 1990s to 2010，891.0 km2 semi-fixed sandy land and 336.3 km² fixed sandy land transformed into drifting sandy land. Desertification process changed significantly mainly distribute in the northern，northwest and southern parts of Mu Us Sandy Land from the late 1970s to the late 1980s，in the western and southern from the late1980s to the late 1990s，in the northwest，western，southern and northeast parts of Mu Us Sandy Land from the late 1990s to 2010，respectively. （5） Over the past 40 years，the average annual temperature showed a gradually increasing trend with an average speed of 0.039 ℃/a，while annual precipitation showed a decreasing trend with an average speed of 0.095 mm/a. And the climate of Mu Us Sandy Land showed a warming and drying trend. Meteorological factors such as temperature and precipitation were those primarily affecting desertification，but not the only factors. Human factors such as population，land use structure and policy implementation should also be the other important factors.

Key words: desertification progress, over the past 40 years, Mu Us Sandy Land