Abstract: A comprehensive and sequential classification system of grasslands (CSCS),a well known grassland classification systems proposed in the 1950s in China,represents a theoretical framework and quantitative potentiality for the science of grassland ecosystem study. At the basic level of a quantitative classification study of CSCS,a considerable achievement has been made and an index chart for determining grassland class has been established. However,owing to the limitations of techniques,relevant reports on quantitative subclass study are infrequent. Based on analysis of the classification subclass indexes this dissertation puts forward the quantitative principles and methods of subclass grasslands,and subclasses classified in Gansu Province. 1) The quantitative index system and index chart for grassland subclass are established; 2) The basic geomorphologic types are divided into 20 types in Gansu Province by extracting the slope,attitude,roughness,mountain and plain factors from SRTM-DEM data; 3) Using supervised classification,36 soil subtypes have been obtained in Gansu Province by extracting the slope,aspect,curvature factors from SRTM-DEM data and soil brightness,vegetation index,NDWI,NDSI and NDMI from MODIS data; 4) 568 subclasses of grassland have been obtained by operating the grid algebra on the grassland first class map,the basic geomorphologic types map and the soil subtypes map.