In this study, a new marine reptile fossil site was discovered in Mountain Daimao, Longgong, Anshun, Guizhou, and preliminary fossil excavation and preparation were conducted on the fossil- bearing site. There were three individual marine reptile in the fossil site, and the preliminary study found that the classification of these fossils was assigned to Sauropterygia. Based on biostratigraphy and sedimentology, it is determined that the fossil- bearing layer is in Anisian, the Middle Triassic and the paleogeographical location is in the southwest margin of the Yangtze Platform. It is the first time that well- preserved marine reptile fossils have been found at the margin of Yangtze Platform in Yunnan, Guizhou and Guangxi area. The discovery and study of this site are of great significance in the evolution of marine reptile systematics in Triassic Period, the reconstruction of paleoenvironment, and the radiation intensity during Anisian Age, the Middle Triassic, and provide new materials for the paleogeographic location.