Abstract: Widely distributed in Yunnan Simao basin, Upper Triassic Waigucun Formation is a red molasses sedimentary containning rare fossils. In the 1∶50000 regional geological surveying process, the authors gathered rich bivalve fossils in southeast of the Simao basin, in which total of 52 species belonging to 21 genera were identified. The discovery of bivalve fossil assemblages supplied the feature of bivalve fauna during the deposition of the Waigucun Formation. According to the stratigraphic ages and characteristics of the fossils of bivalves, the authors suggested that Waigucun Formation was developed during the Carnian-Norian. We made a brief analysis on the evolution of the sedimentary environment during the Late Triassic in the map area.