The Lower Ordovician in Xingshan consists of the Nanjinguang Formation,Fenxiang Formation, Honghuayuan Formation, Dawan Formation and Guniutan Formation.The rock types include limemud limestones, bioclast limestones, oolites, intraclast limestones and dolomites, indicating a typical set of epeiric sea carbonate platform or submerged platform deposits. Grainstones are the main part of the section, among which the oolites, intraclast limestones and most bioclast limestones are cemented by sparry calcite. The grain size distribution and parameters of oolites and intraclastic graistones show that those grains had been transported and agitated during their process of formation. From the beginning of Xilingxia Formation deposition to the end of Guniutan Formation deposition, the sedimentary environments evolved from the tidal flats of restricted platform to open platform and shelf. The tidal flats include dolomitic flat, limemud flat and dolomiticlimemud flat, whereas the open platform mainly consists of grain banks and reefs.