Abstract: At the present time, the Longyangxia reservoir is the largest hydrodynamic engineering at upstream of the Yellow River, Gonghe Basin, where is the important part of the grassland in Qinghai Province. However, the Ecologica-l environment of this area is fragile. The problems such as the grassland degeneration, the land desertification and soil erosion are very serious. Among them, the grassland degeneration is one of the serious problems. All kinds of environment problem of Gonghe Basin are mainly distributed around Longyangxia Reservoir, which is threatening the reservoir and the sustainable development of this area. Based on the widely collecting of data and the analysis of the mult-i temporal and mult-i spectral Landsat TM date with GIS, the "Environment dynamic and monitoring information system of Longyangxia Reservoir Area" is established by the remote sensing technology, in which some software, such as IDRISI, ARC/INFO, ARCVIEW and so on, and PII microcomputer act as main tools and equipment. Supported by this system, the environmental dynamic monitoring and application is efficiently exerted. In order to analyze the process of the present situation of the grassland in this area, these spectral conducting sub-system is utilized and the maximum likelihood is adopted to classify remotely sensed date based on analyzing informationand selecting characteristics. Through knowledge-based classification and modification in GIS, the primary accuracy of classification results is fully improved. This method is an efficient way to extract the easily mixed information and reach the goal of type purification. Supported by GIS, the grassland coverage of this area is detected and analyzed. The result show that from 1987 to 1996, the deterioration of the grassland is very severe, decreasing at the average speed of 37. 4915 km/a (the plants coverage ＞70%) and 9. 1374 km/a (the plants coverage between 30% to 70%). Combined with the plants withering and productivity declination, it directly results in the intensified desertification and soil erosion, and the desertified land is increasing by 21 km² per year. With the use of the analyzing function of the space compounding overlay, the plants coverage chart of the reservoir area in 1987 and 1996 is geometrically overlaid. Together with the use of Bur algebra logical calculation theory, the data of the change on the plants coverage of the reservoir area is gotten from 1987 to 1996.The results show that during this period, the most serious plants deteriorating area is the district with the plants coverage of 70%～90% and 50%～70%, which is relatively deteriorated by 319. 6091 km² and 165.072 km².Though some districts partly have the overturning phenomenon, the whole reservoir area deterioration takes the major position in the area. With information and the field investigation, the main causes of the grassland deterioration are the following aspects：firstly, the increase of arable land, especially the important potential reason; secondly, the overtaking of animal husbandry is the most essential reason.

Key words: Longyangxia Reservoir Area, GIS, Grassland Deterioration, Qinghai