In the Qinghai-Tibet Plateau, a small number of soil samples were collected during surveys of permafrost for the study on building up an index set for predicting thickness of the active layer in the permafrost and for mapping using the digital soil mapping method. By taking into account the relationships between soil and landscape environment, an index set was screened out and established, encompassing, diurnal surface temperature variation, altitude, slope gradient, slope aspect, NDVI and parent rock. With the aid of the method of setting individual samples as representative, a permafrost active layer thickness distribution map of the region was plotted with quite satisfying accuracy and resolution, thus overcoming the difficulty in space extending mapping in the processes of simulating thickness of the active layer of the permafrost with mathematic-physical models and breaking through the limitation of the use of semi-empirical models in mapping being low in resolution. In view of the problem that the sampling sites in the survey were quite limited in number and not so representative of the whole region, the method of modeling based on multi-samplings separately was adopted, instead of the conventional one-time full samples method. Consequently a predictive index set and a map was acquired.