Distributed in the middle section of the Helan Mountains, the strata of Miboshan Formation of the Middle and Upper Ordovician occupy a significant position in the regional stratigraphic sequence and is the most important mark indicating the formation of north-south trending tectonics in the region of the Helan Mountains. This paper studies the sedimentary characteristics, sedimentary sequence, sedimentary lithofacies and palaeogeographic environment of the Miboshan Formation, and divides the strata into three types, namely clastic flows, turbidite flow and normal deep-water sediments. Finally, it makes a new understanding that the north-south trending tectonics in the Helan Mountains started from the middle period of Caledonian and lasted to Cretaceous.