Abstract: In order to study the petrological characteristics and sedimentary environment of the Upper Ordovician Pingliang Formation in the southwest margin of Ordos basin, the petrological characteristics, sedimentary facies, and main controlling factors of the study area were studied by using sedimentology theories and methods based on outcrops, paleontology and grain–size analysis data. The result shows that: ① The Pingliang Formation in the study area can be divided into three lithofacies and two lithofacies associations. ② Based on an analysis of the lithologic character, sedimentary structure, palaeobiologic fossils and grain size parameters, it is believed that the sedimentary environment of Pingliang Formation is mainly deep–water slope. ③ The sedimentary facies is submarine fan, which can be further divided into two subfacies (middle fan and lower fan) and two microfacies (proximal lobe and distal lobe). ④ The main controlling factors of submarine fan include relative sea level, tectonic movement and provenance supply.