Grassland distribution and change dynamics are main objects of desertification monitoring. By using four-period remote sensing images(1976, 1996, 2001, 2006) and under the support of RS and GIS techniques, the spatial information of grassland during the past 30 years in Guinan County, Qinghai Province is interpreted and extracted, and the area, quality and spatial change pattern of grassland are analyzed in this paper. The results shows that: ①The grassland area was decreased by 12.62% in the past 30 years, and grassland degradation is serious, especially for high coverage grassland with a net decreasing rate of 28.87%; During 1976—1996, grassland is mainly converted into cropland, wilderness and forest land, and the converted area of the high coverage grassland occupied 85.9% of total converted area. During 1996—2006, the trend of grassland degradation is slow.②The grassland had greatly degraded from 1976 to 1996, the main reason was that 16.6% of high coverage grassland were converted into middle and low coverage grassland. In the period of 1996—2006, the change of the grassland dynamic degree was small and the area of different coverage grade grassland nearly kept stable. ③The change of gravity center of grasslands is irregular and multi-directional because both natural factor and human actions take effect on grassland in the region.