The soil-forming factors on Xizang Plateau,such as climate,vegetation,topography and hydrographical conditions have changed greatly since the end of Pliocene as the result of uplifting of the Plateau.It exerted deep influence on soil genesis and evolution in this regicn.The field investigation showed that soils bearing composite profile with paleosol layer are widely spreaded.The morphological characteristics of these composite soil profiles may indicate,to a great extent,the polygenetic processes of soils on the Plateau.To give evidence,micromorphological study of soils bearing paleosol layers has been carried out.Thin section observation indicates that the polygenetic characteristics of soils are closely related to the historical natural evolution of Xizang Plateau.