Lots of different ideas stay on whether the Dongbeiwang-Xiaotangshan Fault in the northwest of Beijing exists or how it displays and acts. This paper analyzes the present seismic prospecting data and drill logs from exploration of active faults in Beijing urban areas, and reaches conclusion that the Dongbeiwang-Xiaotangshan Fault does exist as a subsurface fault and it is 40 km long, trending NNE. Divided by the Nankou-Sunhe Fault, the authors name its northeast segment as the Xiaotangshan Fault and the southwest segment as the Dongbeiwang Fault. The Xiaotangshan Fault is active in early Pleistocene but not in middle Pleistocene. The Dongbeiwang Fault is active in the end of mid-Pleistocene but not in late Pleistocene.