Studies of lithostratigraphy and sedimentary environment of the Lower Cretaceous Duoni Formation was made in the Xungmai and Qusongbo" areas, northern Xizang (Tibet). The study area can be divided into western area (Xungmai) and eastern area (Qusongbo) in terms of stratigraphy. The Duoni Formation is divided into three main parts. In the western area (Xungmai). the lower part consists of alternating beds of silty shale, siltstone and fine-grained sandstones?the middle part consists of alternating beds of highly-matured fine-grained, medium-grained and coarse quartzose sandstones; and the upper part consists of intermediate volcanic rocks, dominantly andesites. In the eastern area (Qusongbo),the lower part consists dominantly of fine-grained and medium-grained sandstones; the middle part consists of alternating beds of siltstone and finegrained sandstones; and the upper part consists dominantly of coarse sandstones, sandy conglomerate and conglonmerates. The Duoni Formation is distributed between the Bangong Lake-Nujiang suture and the Yongzhu ophiolite. The Duoni Formation was dominated by a sedimentary environment of barrier bars and beaches in the study area, by a shallow-water marine sedimentary environment to the west of the study area, and by a sedimentary environment of tidal flat lagoons and marshes to the east of the study area. The eastern coastline of the Middle Tethys might lie east of the study area in the early Duoni stage. It extended to the western part of the study area in the middle stage and to the eastern part in the late stage. This conclusion describes the transgression and regression processes of the Middle Tethys during the deposition of the Duoni Formation. This has provided new data and information for the study of the evolution history of the Middle Tethys, the determination of the paleogeographical outlines and the formation of the Qinghai-Tibetan plateau.