Abstract: Shallow marine carbonates dominate the Ordovician of the Helan Aulacugen, with clastic and pyroclastic rocks as the second. In the carbonate deposits, there are micritic limestone,argillaceous limestone, oolitic limestone, calcirudite, crystalline dolostone and argillaceous dolostone. Stromatolite and cloudy limestone occur in some areas. They were formed in a variety of environments, such as the mud-dolomite tidal flat, restricted platform, open-platform, shelf and submarine fan in an succession from land to sea.