The Middle Ordovician Zhaolaoyu Formation is mainly outcropped in the west and southern margin of the Ordos Basin. After a thorough field investigation in 2013, the writers found nearly 30 slump layers and slump debris layers of different scales in the Middle Ordovician (1st~4th Member of the Zhaolaoyu Formation). All the boundaries between the slump layer and overlying/underlying strata are very smooth and distinctive. Most slumping layers have a stable thickness and are widely distributed among the study area. Based on the field observation and the related data of former researchers, the writers conclude that all the slump features in the Zhaolaoyu Formation are the products of ancient seismic slumping caused by tectonic movement (including the volcanism). Other factors such as storms or tides can be excluded for the very deep formation depth of the Zhaolaoyu Formation. Each slump layer is the corresponding to one big ancient earthquake event. The abundant slump layers in the Zhaolaoyu Formation indicate the intensive tectonic activities around the sedimentary basin during the Middle Ordovician. The authors further infer that the “abnormal layers” of limestone breccias in the Zhaolaoyu Formation are probably also the products of sliding/slumping movements triggered by independent seismic events, although they may have little or even no evidence of obvious slump structures.