Abstract: Mangxiang Formation section in Duosang graben of Ngangla Ringco area is described in details. Lithological characteristics, stratigraphic contact relationship and sedimentary environment are also discussed. It is recognized that Mangxiang Formation in Duosang graben are composed of fluvialfan ,river and lake facies from bottom to top and the strata make up a whole sedimentary cycle. Its source area is mainly from the horst of eastern Duosang graben. Some fossils of bivalve and plants were discovered in Mangxiang Formation of Duosang graben. And a Psilunio stoliczkai-Rhododendron cf. namalingense assemblage zone has been established. The formation time of Mangxiang Formation belongs to Miocene according to fossil assemblage and ESR age determination.